Chapter 1. First Trimester: Normal and Variations

Women’s Point of Care Ultrasound

Welcome to the chapter on first trimester pregnancy – normal and variations.

This chapter will begin by discussing the reasons for performing a first trimester scan, as well as describing methods and measurements. It will also review some of the most common variations seen at this gestation.

The chapters to follow in this course will be more specific in describing complications of the first trimester.

Objectives:

The objectives of this chapter are:

List the indications or reasons for performing a point-of-care ultrasound in the first trimester of pregnancy

Learn the importance of correlating BhCG levels (when available) with what is seen on ultrasound, rather than relying solely on either one

List the essential components of a first trimester ultrasound

List the sequence and average gestational ages of when these structures are seen on ultrasound (gestational sac, yolk sac, fetal pole, and fetal heart rate), and how to adequately assess or measure each.

According to ISUOG (International Society of Ultrasound in Obstetrics and Gynecology) the main goals of performing a first trimester ultrasound is to facilitate optimal care and achieve the best possible outcomes for mother and baby¹. There has been some controversy about the need for routine first trimester ultrasound in normal pregnancy. If a woman is only allowed one ultrasound in the pregnancy (because of limited access or payment concerns), some feel that the second trimester anatomy scan is of more value than a first trimester exam. Many women will get a first trimester scan primarily for the purpose of obtaining accurate dates of gestational age. The primary reason for doing a “dating scan” is that clinical decisions throughout the rest of the pregnancy are based on accurate pregnancy dating. And the most accurate time of pregnancy to establish gestational age is early in the first trimester. There are also pregnancy complications that can be recognized in the first trimester that have implications for pregnancy outcome.

Indications

Indications for first-trimester sonography include, but are not limited to^:1,2,3

1. Conducting routine assessment (screening) of low-risk pregnancy

2. Confirming presence of IUP (intrauterine pregnancy)

3. Estimation of gestational (menstrual) age

4. Determining viability by cardiac activity

5. Determining gestation number, and amnionicity/chorionicity of multiples

6. Evaluating a suspected ectopic pregnancy

7. Evaluating a suspected hydatidiform mole

8. Reconciling discrepant uterine size for gestational age

9. Determining causes of vaginal bleeding

10. Evaluating pelvic pain

11. Evaluating maternal pelvic masses and/or uterine abnormalities

12. Removal of IUD in early pregnancy

Components

When a first trimester ultrasound is done, the following structures to observe include:

1. Gestational sac

2. Yolk sac and membranes

3. Embryo/Fetus

4. Crown-rump length

5. Fetal heart motion

6. Adnexa and uterus

Figure 1. First trimester exam components.

POCUS First Trimester

The first trimester POCUS is not intended to replace a complete first trimester evaluation. Often the scan is done solely to establish viability and measure a simple crown rump length. It is considered a “limited” exam but needs to be done in a thorough and systematic manner to be able to rule out abnormalities.

In some areas this may be the only ultrasound that a woman will receive in her pregnancy, so it needs to be as complete as possible.

AIUM defines a limited or POCUS exam as a limited exam performed to invetigate a specific question.¹AIUM also gives a list of other conditions to evaluate during pregnancy as part of a limited exam.¹ Limited exam components for a second/third trimester exam include evaluating interval growth, estimating amniotic fluid volume, evaluating the cervix, and assessing the presence of cardiac activity. These will be reviewed in the Second/Third Trimester POCUS course.

Comment

The first trimester POCUS exam is considered a limited exam, even though it may have most of the components of a “complete” early pregnancy ultrasound. Most ultrasound societies suggest that a POCUS first trimester exam should not replace a standard or complete first trimester exam. However, if standard or complete ultrasound is not available, the POCUS remains valuable. The POCUS scan would be done in response to the indications listed above.

The first element of the exam is to determine viability of the pregnancy. If there are symptoms of impending pregnancy loss or complication, observing a fetal heartbeat is a critical discovery if the gestation is far enough along for ultrasound to see it.

Often, women will have pregnancy lab results (such as urine or blood testing) prior to an ultrasound. Serum pregnancy testing (quantitative BhCG) provides some information, but ranges of normal are too wide to apply for confirming a gestational age without correlating it with an early ultrasound measurement.

Confirming Pregnancy

Expected findings on Ultrasound by Menstrual age ^1,3,4,5

The first estimation used might be her last menstrual period (LMP) start date if it is known for certain, and she has regular cycles, or ovulation history. For most women who know their own history, the ‘menstrual age’ is fairly reliable in determining her gestational age. From that information correlated with the BhCG and ultrasound imaging, one can determine if the pregnancy is progressing normally.

Why is confirmation of pregnancy in the first trimester Important?

--It establishes viability and location of the pregnancy

--It establishes gestational age.

The earliest measurement of a crown rump length is the MOST accurate measure of the gestation of pregnancy. Why is this so? Normal embryos/fetuses are all the same size early in pregnancy. In later gestations, fetuses can grow at varying rates and become either large or small for gestational age, depending on genetic predisposition, growth potential, and maternal factors. Precise confirmation of gestational age after the first trimester is less accurate (due to a wide margin of error). In other words, size does not always correlate with gestational age as pregnancy progresses. The due date should never be changed later in the pregnancy, even if the size calculation suggests a different gestational age. The measurements must always be compared to the earliest ultrasound, which is the most accurate. A discrepancy would be better interpreted as an indication of a fetal growth abnormality rather than a different gestational age.

Critical Clinical Decisions

Why is using an early measurement important?

--POCUS is the best method for estimating delivery date⁶

Critical decisions may be needed for intervention later in the pregnancy and the most accurate due date is important, if not essential, in some cases.

--Determining predicted survivability of a preterm infant

--Determining when to induce labor safely

--Determining when a pregnancy is possibly preterm or post-dates, to begin antenatal fetal testing

NOTE: The next 2 sections below pertain to clinicians who have access to laboratory testing. Many women’s health providers work in more remote locations without access to this testing. For those providers, it is still helpful to review this material in case a woman brings lab results from another facility. It also serves to reiterate the importance of performing an early POCUS exam for women to confirm gestation. Many women in remote areas have little to no prenatal care, and a POCUS provides valuable information.

Detecting and Confirming Pregnancy with urine testing

Figure 2. Urine testing for pregnancy.

Before discussing ultrasound evaluation, the clinician needs to recognize other ways of testing for pregnancy, and what these tests mean.⁷

A URINE test is QUALITATIVE – meaning it is either positive or negative and does not give a numeric value.

A urine test typically becomes positive when there is a threshold of about 20-50 mIU/mL Beta hCG.

There is a wide variation in sensitivity – the test can be positive with BhCG’s as low as 6-12mIU/ml. It is usually positive by a day or two after a missed period, and it may be faintly positive a day or two before then.

False negative causes:

--Measurement too early after conception

--Diluted urine specimen

False positive causes:

--Blood or protein in urine; high urine pH or presence of semen

--Some ovarian tumors

--Drugs (aspirin, methadone, carbamazepine)

Detecting and confirming pregnancy with serum testing

Figure 3. Serum testing for pregnancy.

A serum or blood pregnancy test is QUANTITATIVE, measuring a numeric level of the hormone found in the bloodstream. It can detect BhCG levels as low as 1-2 mIU/mL .

Typically, any value less than 5 is a “negative” for ruling out pregnancy and a value above 25 confirms a pregnancy.

NOTE: It should be mentioned that there are also serum QUALITATIVE tests, which, like the urine test, only detects the early presence of BhCG and not the amount. This test is useful in some circumstances, such as a rapid rule out of pregnancy prior to a procedure)

The earliest gestation that a pregnancy can be confirmed by ultrasound depends on the type of machine used, the skill of the provider, and the maternal body habitus. Typically the numbers used for this are when BhCG levels are between 1,500 and 2,000.

BhCG levels reference ranges⁸

Table 1. Increase and decrease of BhCG levels correlating with gestational age.⁸

For example, drawing a single BhCG level does NOT confirm a woman’s gestational AGE. If her BhCG levels are 25,000, this could mean she is anywhere from 6-16 weeks based on the reported ranges for these levels (see Table 1 above). So an ultrasound would be much more precise than a BhCG level in confirming the number of weeks, since the margin of error of ultrasound is only a few days. This is true once the pregnancy can be seen with ultrasound.

False positive and False negatives⁹

False positive results can be reported from women producing aberrant forms of hCG that are not biologically active but will cross-react with the hCG assays.

Limitation of BhCG assessment is that early levels can be affected by presence of heterophilic antibodies in some women that interfere with the assay detection of BhCG.

False negatives can be due to testing too early in the pregnancy. This can be common due to a woman conceiving at a different point in her cycle, especially if she has long cycles (intervals of 32 or more days between menses).

In the above cases, there are other forms of serum testing that can be performed by labs, if available.

Why is it important to know the BhCG levels if they are not precisely predictive, and how does the reference table help with clinical decisions?

-- Trending of SERIAL BhCG LEVELS (whether increasing or decreasing) can direct management of the pregnancy, such as determining if a pregnancy is failing or if it is ectopic.

Discriminatory zones ¹⁰

Ranges of Beta hCG levels, correlated with each week of pregnancy, are very wide (see Table 1.) There is no direct correlation between a single BhCG and a particular week of gestation.

HOWEVER,

It CAN be used generally to determine what might expect be seen by ultrasound. For example if a woman has an initial BhCG of 400, one would NOT expect to see a gestational sac in the uterus with ultrasound. In that case, if she was bleeding, the best follow up would be to repeat the BhCG rather than try to look with ultrasound..

It also CAN be used as a baseline for serial tracking when there is a suspected abnormal pregnancy. Subsequent tests would demonstrate increase, plateau, or decrease, any of which could predict the outcome of the pregnancy.

It can be of help in determining the presence of a complicated pregnancy, such as early pregnancy loss, ectopic, twin gestation, or trophoblastic pregnancy

There are discriminatory zones used to make clinical decisions on pregnancy viability associated with ectopic pregnancy.¹¹ This will be described more completely in the Chapter on PUL/Ectopic pregnancy.

Although BhCG levels alone should not be used to diagnose an ectopic pregnancy, it should be eventually correlated with ultrasound findings once the gestation is far enough along to be seen with ultrasound. Accurate gestational age by ultrasound calculation, rather than an absolute hCG level, is the best determinant of when a normal pregnancy should be seen by a transvaginal ultrasound¹¹.

Discriminatory Zones¹¹

The ‘discriminatory level’ refers to the hCG value above which a normal intrauterine gestation should be detected by ultrasonography. For example, if there is no gestational sac when the hCG measurement is above the discriminatory level, this suggests a nonviable gestation (EPL or PUL).

In 50-70% of these cases, findings are consistent with an ectopic pregnancy, so there should be a high suspicion to rule this out if the levels are higher than expected for not seeing an intrauterine GS. Some studies suggest that levels used to make decisions should be conservatively high to avoid the potential of misdiagnosis and possible interruption of a normal IUP. Multiple gestations have higher levels of BhCG at any gestational age, so high levels without seeing a sac in the uterus could be a clue that the pregnancy might be a multiple gestation rather than an ectopic.

NOTE: Pregnancy of unknown location (PUL) is not a diagnosis but is treated as a suspicious finding, while waiting to confirm the precise location of the pregnancy (IUP or ectopic).

NOTE: A single HCG cannot diagnose either viability or pregnancy location.¹²In cases where an IUP cannot be identified with reasonable certainty in the uterus, serial hcg and subsequent ultrasound exam are usually required to rule out ectopic.

Serial measurements of BhCG

When tracking serial B-hcgs, there should be a curvilinear increase in levels until it plateaus at about 100,000 by 10 weeks’ gestation.

A slower increase in levels may indicate a failing pregnancy, especially if the levels are below the threshold at which a pregnancy might be seen by transvaginal ultrasound (TVUS). The typical expectation is that a quantitative B-hCG would roughly double in 48 hours from the initial value. However, there are variations to keep in mind that could still indicate a normal pregnancy.

The minimal expected increase in BhCG in 48-hours is as follows:

--at least 49% increase if initial hCG is 1,500 mIU/ml

--at least 40% increase if initial hCG is 1,500-3,000,

-- at least 33% increase if initial hCG level is greater than 3,000 mIU/ml.¹¹

Decreasing hCG values suggest an early pregnancy loss, but this should not be considered diagnostic, especially if it occurs prior to being able to confirm a gestation with ultrasound..

Approximately 95% of women with a spontaneous loss will have a decrease in hcg concentration of 21-35% in 2 days, depending on initial Bhcg levels. If there is a decrease, with no evidence of an IUP, the woman should be carefully monitored until nonpregnancy levels are reached, since the location of the pregnancy may still not be known.

To determine if falling BhCG levels are from a very early pregnancy loss or from ectopic, contents of a uterine aspiration can be sent for pathology studies. If chorionic villi are found in the removed tissue, this confirms that the pregnancy was in the uterus ( EPL) rather than an ectopic.

A plateauing of Bhcg means that evacuation was incomplete (either spontaneous, induced, or surgical) and that further treatment is needed. This also can be an indication of a PUL or ectopic. Plateauing of the numbers can also be an indication of a condition known as a chronic ectopic. Large decreases in hcg after uterine evacuation are more consistent with failed IUP rather than ectopic.

NOTE: A single Bhcg or ultrasound may not be sufficient to diagnose EPL¹². However, in cases where an IUP cannot be identified with reasonable certainty in the uterus, serial hcg and ultrasound exam may be required before initiating treatment for a suspected ectopic.

When can you first see a pregnancy with ultrasound ¹⁰

Table 2. Early pregnancy components seen by transvaginal and transabdominal ultrasound.

¹⁰

Table 3. (below) appears complicated, but the following examples gives understanding to how the numbers can be used.

Table 3. BhCG levels for predicted probabiity of visualizing structure with transvaginal ultrasound

¹⁰.

Looking at OLDER studies, it was said that the earliest you could recognize a gestational sac with a transvaginal ultrasound (TVUS) was when the BhCG level was between 1,000 and 2,000, typically using the values from 1500 to 2000 mIU/ml.

Newer data¹⁰ suggests that even as early as BhCG of 879, you have a 50% chance of seeing a gestational sac (GS), using a machine with great resolution. At a level of 3,500 there is a 99% confidence that a gestational sac will be seen in a normal pregnancy using a TVUS. The number 2,317 BhCG is the highest level you could go to and NOT see a gestational sac with the pregnancy still being normal.

Table 4. (below) correlates mean sac diameter, age in wks, and BhCG levels.⁴

Table 4. Correlation between MSD, gestational age, and BhCG levels.⁴

When can you first see a pregnancy with ultrasound: YOLK SAC

When looking for a yolk sac (YS), you might see it as early as a BhCG level of 1,826. But at a BhCG level of 17,616, you would EXPECT 99% of the time to see a yolk sac in a normal pregnancy. The number 17,616 would be the highest BhCG level you could get to and still possibly not see a YS, even in a normal pregnancy.

When can you first see a pregnancy with ultrasound: FETAL POLE ³

For measuring fetal pole length, the earliest you would expect to see it with a TVUS is when the BhCG is 10,091.

But at a BhCG level of 47,685, you would EXPECT 99% of the time to see a fetal pole. If you have a BhCG of 35,486, you may still not be able to see a fetal pole, yet it may be still a normal pregnancy.

With transabdominal ultrasound you could expect to see a GS and/or small embryo by 6-7 weeks gestation, depending on the maternal body habitus and sophistication of the ultrasound machine.

Table 5. Expected findings on ultrasound b y menstrual age.³

When can you first see a pregnancy with ultrasound: EXAMPLE

A woman presents with bleeding and a positive pregnancy test. A serum pregnancy test is helpful to decide whether she is far enough along in the pregnancy to benefit from an ultrasound, or whether it is too early to see any structures.

Her serum BhCG is 15,800.

What should be seen by TVUS?

A gestational sac? Yes

A yolk sac? Probably, since the highest number of BhCG where a YS may still not be seen is 9,975. However, the number to reach so you can have highest confidence would be a BhCG of 17,616 (99% confidence).

A fetal pole? Maybe yes, maybe not

At a value of 15,800 this exceeds the number for seeing a fetal pole in 50% of women, but it does not reach the number where it can be seen with 99% confidence (47,685). Nor does it reach the number 35,486, where you would be suspicious of an abnormal pregnancy if a fetal pole was not seen.

While using these tables, keep in mind that the numbers in parentheses are also ranges of potential variation in these numbers. So even with this table, there can be considerable variance.

The conclusion here is that a single BhCG number is not as valuable in determining gestational age.

Imaging Early Pregnancy Components

Scanning method

The preferred method for scanning in the first trimester is TRANSVAGINAL. The higher frequency produces 40-50% improvement in resolution over transabdominal, This provides much more detail, which is critical in detecting a tiny sac or fetal pole, and very important for looking at the adnexa to rule out ectopic pregnancy.

Use of TRANSABDOMINAL ultrasound is acceptable, but it does require a full bladder. Even with a full bladder, views of small gestations are limited. Large maternal body habitus further limits the ability to see pelvic structures. Another disadvantage is a retroflexed uterus that is obscured by the pubic bone. This prevents the clinician from being able to confirm the presence of a GS.

Figure 4. Scanning method for first trimester POCUS.

GESTATIONAL SAC ^1,3,4,5

The first structure to be seen by ultrasound in early pregnancy is the gestational sac (GS). It is expected to be seen at 5 weeks, or when the sac is 2-5 mm in size.

The sonographic characteristics are that GS’s are hypoechoic (or anechoic), and circular or ovoid.

The way to measure the GS is at the inside chorionic margin in all 3 diameters. That number is then averaged to get the number to record

Usually the ultrasound machines calculate this, and some calculate it with just 1 or 2 diameters rather than 3. Most ultrasound societies, however, recommend 3 separate measurements of the GS diameter to arrive at an average. This mean sac diameter, also known as the ‘MSD’.

A formula that can be used to calculate gestational age by using the MSD is:

a (embryo’s age in days) = d (diameter in mm) + 30

Figure 5. Principles for measuring gestational sac.

The mean sac diameter (MSD) measurement is used to determine gestational age before a crown rump length (CRL) can be clearly measured. The gestational sac can be measured from 5-6 weeks up to 11 weeks, however once a fetal pole is seen, the CRL is more accurate.

An important concept is that the mean sac diameter of 25 mm or greater with no visible embryo suggests failed pregnancy. The chapter Early Pregnancy Loss in this course reviews this in greater detail.

The endometrium is also important to notice. It is normally thick and hyperechoic (whiter, indicating increased density)

At 5.0 weeks a gestational sac should be seen.

Figure 6. Gestational sac, Image 1.

At 5 ½ weeks, you should see a YS and the gestational sac. You may also see a flickering of a heart beat at a location near the YS where the fetal pole is located. But the fetal pole may not be distinct enough to measure.

At 6.0 weeks, you should see a GS, YS and an embryo with a length less than 5mm. You should definitely see fetal cardiac motion at this time, as well.

After the fetus measures > 5mm, the estimated gestation will be determined by the ultrasound’s computer, using that length to interpret the gestational age. If you are using an ultrasound machine that DOES NOT have obstetric calculation capability, the actual length in mm or cm can be compared to printed standardized tables listing fetal size and gestational age correlations.

If the measurement number by ultrasound in millimeters is in agreement with the gestational age based on the LMP date (by 5 days or less), this confirms the LMP date.

This information is useful to detect discrepancies in gestational age. Discrepancies in gestational age determination are VERY common and are discussed at the end of this chapter.

Image 2. Early gestational sac TVUS

The images below (Images 3 & 4) are early pregnancy images. These images demonstrated a very early gestational sac inside the endometrium. Notice the thickness of the endometrium – this is a normal and reassuring sign that the endometrium is rich in blood supply and tissue for the embedded embryo.

The image on the right shows that a transabdominal view is less clear, and makes it difficult to tell if the dark area (indicated by the arrow) is actually a gestational sac.

Images 3, 4. Early IUP comparing TVUS and TAUS imaging.

Figure 1 (below) shows a diagram illulstrating how to measure the gestational sac – from inner edge of one side to the inner edge of the other side. Typically the ultrasound machine will only give you a computed gestational age if you have measured in 2-3 dimensions. Some machines, however, will display a gestational age with a single diameter measurement. To be more accurate, it should be measured in 3 dimensions.

Figure 1. How to measure the gestational sac.

Mean sac diameter measurement ^1,3,11,12

Images 5 and 6 (below) show the gestational sac seen clearly on the left, with probable YS or embryo in the right image (blurred lines inside the sac).

The other thing to note is that one image is perfectly round, and the right image is bean-shaped. In general, it is reassuring to see a smooth round or ovoid shape to the gestational sac as an indication of a healthy gestation.

Images 5-6. Images of early GS in 2 views.

As a review, measurement includes a mean of making orthogonal sac dimensions (inner-to-inner diameter measurements in mm) THREE TIMES, then using the average. Cursors should not be placed on decidual area.

The gestational sac in the image below is seen clearly on the left, and there is perhaps a YS or embryo in the right image (blurred lines inside the sac).

The other thing to note is that one image is perfectly round, and the right image is bean-shaped. In general, it is reassuring to see a smooth round or ovoid shape to the gestational sac as an indication of a healthy gestation.

The table below (Table 6) is a quick review of how to correctly measure a first trimester gestational sac. This table defines precise measurement of a mean sac diameter.This table defines precise measurement of a mean sac diameter.

Table 6. Mean sac diameter measurement.¹⁶

Keep in mind the margin of error for dating using GS measurements is from 4-11 days.

As a review, measurement includes a mean of making orthogonal sac dimensions (inner-to-inner diameter measurements in mm) THREE TIMES, then using the average. Cursors should not be placed on decidual area.

NOTE: Once a crown-rump length (CRL) is measurable, you should NOT measure the GS, since it can skew the dates based on the CRL. At that point the CRL measurement is more accurate than the GS.

Images 7, 8, 9. Early IUP gestational sac measurements.

The above images (Images 7,8,9) illustrate the use of 2 measurements of the gestational sac to get an accurate measurement. These were taken using a transvaginal ultrasound. The image on the left has 2 measurements (length and depth), and the right shows the view of how to find width by rotating the probe 90 degrees.

Finding a gestational sac in a retroflexed ut

One important thing to remember is that a pregnancy in a retroverted or retroflexed uterus may be much harder to see. So be certain that the uterine position is assessed as you are scanning to optimize your view of a gestational sac. It may be necessary to use a transvaginal probe to visualize the early pregnancy in a retroflexed uterus. Use of a transabdominal probe is limited in this case and filling the bladder will only push the uterus further back. Sometimes emptying the woman’s bladder will allow the uterus to rise forward, but this is not always helpful. If a transvaginal probe is not available, the woman may need to be referred to a provider with a TV probe.

This image below (Image 10) is one of an acutely retroflexed uterus with a small gestational sac. Remember that with a retroflexed uterus, the fundus is angled toward the right of the screen. The gestational sac is a bit difficult to see. If this image was attempted by transabdominal scan, it is likely that the uterine fundus would not be seen due to how deep in the pelvis it is deflected.

The red line outlines the uterus, and the gestational sac is indicated with the annotations.

Image 10. Visualizing a gestational sac in a retroflexed uterus.

Decidual sign

Once a gestational sac is seen, it is important to look at the area surrounding the sac. This should appear as a halo-like ring. It may be very bright, or it could be subtle, and just a bit brighter than the area around it. This is called the decidua and it represents where the pregnancy has embedded into the endometrial tissue.

Image 11. Decidual reaction early IUP, image 12.

.

Double Decidual sign

Figure 7. Double decidual sign. Image 12

A reassuring finding when observing the decidual reaction is what is called “ double decidual sign” (DDS) or intradecidual sign (IDS). This is where a brighter echo is seen closer to the gestational sac, and a separate less bright halo surrounds it. This is not visible in all pregnancies, but when it is seen, it is a confirmation that a pregnancy is present, and not just a pocket of fluid.

If there is just a dark hypoechoic circle without any decidual reaction around it, this could represent a “pseudo sac” or false sac. It represents a small collection of fluid and not a pregnancy. This can be a concerning sign if you are confirming a pregnancy and do not see a good decidual response around what you think is the gestational sac. Seeing a “pseudo sac” raises the suspicion of a pregnancy located outside the uterus, or ectopic. Recent opinions on pseudosac is that it is not always a sign of ectopic, and should not be used as a criteria for making that diagnosis.

Absent Gestational sac¹²

Figure 8. Characteristics of absent gestational sac.

A NON-reassuring finding is inability to see a gestational sac in the uterus when it should be seen. As mentioned earlier, a sac should be visible at about 4.5-5 weeks (menstrual age) with a TVUS, and somewhere between the BhCG level of 800 to 3500.

If the BhCG level is less than 1,000, it is difficult to see the sac for certain.

Here are some additional findings to look for:

--if the GS is greater or equal to 8 mm, and you do not see a yolk sac, this is abnormal

--if the GS is 20-25mm and there is no fetal pole, this is abnormal.¹²

Early pregnancy loss will be covered in more detail in the next chapter.

YOLK SAC AND AMNION

The next structures that can be seen in early pregnancy are the yolk sac and the amnion.

Figure 9. Characteristics of early YS and amnion.

The yolk sac

--Expected at 5.5 wks

--Located outside the amnion

--Echogenic ring with anechoic center

--Shape – typically round, but irregular shape may also be a normal variation

--Size – large size may indicate failed pregnancy. Some machines provide calculations for measuring the YS, but this is not typically part of a POCUS exam

--Sometimes seen at the end of a stalk called the vitelline duct

The amnion

--Barely visible at 7 wks as a thin wispy line.

--Separate from the chorion; they remain separate until 14-16 wks, after which they are fused.

Embryonic Anatomy

This diagram depicts the anatomy of a first trimester embryo, much of which can be detected on ultrasound.

The embryo is in the center, encased in the amnion, a thin membrane that is often not visible until about 8 weeks. This amniotic sac is encased within the chorionic cavity, surrounded by the chorion (the structure that invades the endometrium for blood supply). There is usually a separation between the amnion and the chorion in the first trimester. These 2 layers eventually fuse around 14 weeks’ gestation, but it is difficult to see the two as separate after 12 weeks.

NOTE: the yolk sac is located OUTSIDE the amnion, but inside the chorion (between the two layers).

The YS has a number of important functions, including nutritional support and gas exchange between the mother and embryo. It is the main organ of embryonic blood cell production, as well as producing stem cells, germ cells and protein synthesis. It also contributes to the formation of the umbilical cord. The chorion develops from the yolk sac, and the chorion itself is involved in gas and nutrient exchange due to its vascularity. The yolk sac continues to grow during weeks 5-10 of the pregnancy. It eventually disappears at the end of the second trimester, when placental function takes over.

There is a stalk, called the vitelline duct, that connects the fetus to the YS. It is the port of circulation to the embryo.

The amnion forms in the second week of fetal development, and is responsible for surrounding the embryo with amniotic fluid, which supports the development and movement of the embryo.

One important fact to remember is that if you see a YS in the uterus, this means that the pregnancy is in the uterus, even if you don’t see a fetal pole.

Below (Image 13)is another image showing the relationship of the early structures: the yolk sac is seen outside the amnion. The space between the amnion and chorion (called the chorionic cavity or chorionic space) is easily seen in early first trimester.

Image 13. Early first trimester structures.

Below is an image of the vitelline duct connecting the YS (Image 14.1) to the fetal pole (not seen in the image). It resembles a balloon on a string.

Image 14.1 Image of vitelline duct and YS.

The illustration in Figure 10, below, is a graphic representation of the relationship of the yolk sac, amniotic sac, and chorion (which attaches to maternal side).

As seen in the previous drawing, the yolk sac is outside the amnion but inside the chorionic space.

Figure 10. Illustration of early gestation components.

EMBRYO OR FETAL POLE

Even though the definition of “fetus” refers to a gestation after 7-8 week, the term “fetal pole” is still used to describe what is seen and measured earlier than that gestation. A fetal pole is expected to be seen at 5.5 to 6 wks’ gestation with TVUS. Cardiac activity should be observable when the embryo is 2 mm or greater.

The embryo first appears as a “grain of rice” attached to the YS. This is sometimes referred to as the “diamond ring” sign.

Figure 11. Diamond ring sign in early gestation.

Image 15. Small fetal pole adjacent to YS.

The round ring in Image 15 is the yolk sac, and just below it is the “blur” that is the fetal pole. If you were scanning this live, you might be able to detect a small flicker of a heart beat inside this “blur”.

Also noted in this image is a reassuring sign of a bright decidual ring.

Fetal Pole and Yolk Sac (2 images)

When it appears that the fetal pole and the YS are about equal in size, this is referred to as the “double bleb” sign. The way to tell which is the embryo and which is the YS, note that the YS is a ring with a hypoechoic center.

Images 16, 17. Double bleb sign - YS adjacent to fetal pole.

Image 18 (below) is an old, unclear image of an early gestation. The red line outlines the long view of the uterus. The decidual ring looks very smooth and homogeneous, and there is a small embryo measuring about 5 wks 6 days. Color flow was applied (you can see the blue and red dots throughout the myometrium) to see if a heartbeat can be detected. But if you look at the lower right of the screen you can see that the gestational age by measurement is 5 wks and 6 days (just short of 6 wks). At this gestation, it is not always possible to see a heartbeat. This is especially true if you are using an older machine with less resolution.

Image 18. Fetal pole at 5wks 6 days, without color flow.

Image 19 (below) is of an embryo at 5 wks and 5 days. The tiny 3D model of what the fetus actually looks like is on the white triangle. The triangle is meant to depict how the ultrasound slices through the embryo to get the image on the ultrasound.

Image 19. Illustration of real size of 5w5d gestation compared to ultrasound image.

Below in image 20 is an image of a fetus at 7 weeks, and it demonstrates more definition and echogenicity.

Image 20. Fetal pole at 7 weeks.

In Image 21 there is a view of an embryo at 7 wks 1 day, and the 3D model on the white triangle depicts what the fetus actually looks like, and how the ultrasound beam slices through the embryo to get the image.

Image 21. Illustration of real size of 7w1d gestation compared to ultrasound image.

The illustration and image below (Figure 12) depict the fetal pole as it begins to grow. You can easily see the amnion encircling the embryo, and the vitelline duct and YS in the chorionic space. There is a good decidual reaction noted also. At about 8 weeks, you can begin to differentiate the fetal trunk from the fetal head. At this point the embryo is now called a FETUS, and no longer an embryo.

Figure 12. Illustration of layers within GS and relationship to fetal pole. Image 22.

Image 23 is a 3D rendering of an early gestation further illustrating the yolk sac attaching to the fetus via the vitelline duct.

Image 23. 3-D Image of early gestation highlighting vitelline duct and YS.

Measuring the Embryo/Fetus

Crown-rump length (CRL) is the term used and is essentially the same as measuring the fetal pole.

Measurement is taken along the long axis of the fetus

Typical measurements:¹³

8-14 mm at 6 wks

14-20 mm at 7 wks

21-30 mm at 8 wks

NOTE: The CRL is THE most accurate biometric measure for predicting gestational age

Margin of error: +/- 3-8 days

Biologically reproducible

Not influenced by any extrinsic variables (like genetics or growth potentials)

Measuring the early embryo/fetus

This chart is a quick review of how to correctly measure a first trimester embryo/fetus, defining precise measurement.

Table 7. Measuring the early embryo.¹

In review, the accuracy of dating using CRL measures is within 3-8 days’ margin of error.

Measurement is obtained by making the longest straight line length of the embryo from the outer margin of the cephalic end to the rump. Neck position should be neutral (not flexed or extended). It is recommended that the most accurate CRL is one based on the average of several serial measurements. The sonographic standard for this is 3 separate documented measurements.

Figure 13. Description of crown rump length as the most accurate biometric determinant of gestational age.

This is an additional image of an early CRL at 6.5 wks. Just below the fetal pole, you can see the thin wisp of the amnion.

Image 24. Embryo at 6w5 d.

This image (25 below) at 7wks 0 days gestation shows a well-defined fetus, along with a vitelline duct connected to a well-defined yolk sac.

Image 25. Embryo at 7wks 0d.

Below is another view of a 7wk 1 day gestation. The image is very clear since it is used with a more sophisticated ultrasound machine having excellent resolution.

Image 25.1 Embryo at 7w1d

In this image below at 8wks 5 days, limb buds are now seen, and there is a definite distinction between the fetal head and body (commonly first seen after 7.5 to 8 weeks’ gestation).

Image 26. Fetus with limb buds.

It is at this gestation (7-8 weeks) when the term ‘fetus’ is used instead of ‘embryo’.

FIRST TRIMESTER CARDIAC

Although detectable cardiac contractions begin when the fetus is at 5 wks and 2 days from the LMP, the heart motion is not always observed until 6 weeks menstrual age.

Typically, if the embryo measures 2-4 mm, you will be able to see a heartbeat about 80% of the time

If the embryo measures 7 mm, you should see a heartbeat 100% of the time.

M-mode is what is used to measure fetal heart rate. M-mode allows us to document fetal heartrate with an ultrasound image, and it should be captured as an image to store in the woman’s medical record along with the other images. If the pregnancy fails after this, you will at least have had the documentation as to when the embryo was still viable.

Figure 14. Fetal cardiac contractions

When the flickering heartbeat is seen and captured on M-mode, the rate should be in the rage that is normal for the gestational age.

If the embryo or fetus has a heart rate <90-100 bmp, this is associated with a poor outcome. A rate lower than 90 has been associated with impending fetal loss.

NORMAL fetal heart rate (FHR) is above 120, but 100-115 is common prior to 6 wks

NORMAL FHR is 144-170 by 8 wks

Figure 15. Embryonic bradycardia

NOTE: **DO NOT USE COLOR OR SPECTRAL DOPPLER in first trimester before 10 weeks’ gestation. This has the potential of generating too much energy toward the tiny embryonic tissues and carries potential for harm. The exception is when trying to rule out early pregnancy loss, when use of color flow enhances the diagnosis of viability by its ability to detect blood flow into the embryo.¹⁴

If no cardiac activity is visible in an embryo by 7mm in length, embryonic demise should be suspected, and a TVUS should be repeated in one week to confirm.

MMode (motion)

Figure 16. M-Mode example. Image 27.

This is what M-Mode looks like.

When M Mode is selected on the ultrasound machine console, it displays a split screen. One screen will continue to show the 2D image of the fetus/embryo. There will be a moveable vertical line that can be moved with a track ball or finger on a track pad or touchscreen, to make the line intersect with the part of the fetal heart that has the most motion while the heart is beating. Once this is done, the M-mode selector is pressed again, and it creates an ongoing recording of the fetal heart by creating “blips” on the screen (similar to an EKG).

To measure the blips, you select FHR (fetal heart rate) from the measurement menu, and then select a starting and stopping place on the screen. You will see very tiny “blips”, that are often hard to discriminate in early pregnancy. The line with the blips will be at approximately the same depth on the M-mode screen as it is in the 2D screen (indicated by the green arrows in this image). Some machines have settings that show the heart rate by measuring the distance between one “blip” and the next, while others are set to measure heart rate using the distance between 2 “blips”.

Figure 17. M-Mode example 2, Image 28.

Above (Figure 17) is an earlier gestation, so in order to view the heart to capture its beating, the image has to be enlarged before engaging the M-Mode. This allows the machine to detect heart motion more readily. Also, the “blips” will be tinier and harder to see.

TIP: If the fetal heartrate is difficult to visualize, use the Zoom or Depth mode on the computer to increase the size of the fetus before applying M-mode.

If you are unable to measure the “blips” accurately with the M-mode, it is acceptable to write in your report that FHR was seen but not imaged, and to document the observed rate for documentation.

Late First Trimester Exam Components

In late first trimester there are other parameters to notice as the embryo grows and becomes a fetus.

The head is now distinguishable from the body. There is a hypoechoic region in the back of the head or hindbrain that is a normal structure, called the rhombencephalon. This develops into the pons and cerebellum. It should be recognized as a normal finding and not an absence of brain tissue.

There is often a bulging from the fetal abdomen into the umbilical cord, called midgut herniation. This is also normal. This typically moves back into the abdomen around 11-12 weeks gestation

Also, at about 11 weeks, one can see definite ossification of bones.

Figure 18. Late first trimester characteristics.

At this point you may also see small limb buds, and some spontaneous fetal movement or wiggling.

Image 29. Rhombencephalon and embryonic foregut.

Images 30, 31 demonstrate a bit of midgut herniation (sometimes called embryonic foregut), as well as a rhombencephalon.

Images 30,31. Early gestation exaples of rhombencephalon, amnion, midgut herniation, and YS.

Nuchal Fold

At about 8-10 weeks, more internal structures will be noticed, including lines that appear around the fetal body. In the image on the right in Images 32,33 , it appears that there is a bulging of the nuchal fold, the skin in the back of the neck. If it seems to be enlarged, it might indicate conditions such as cystic hygroma or Down’s syndrome.

Images 32,33. Ultrasound image of nuchal fold.

As a point-of-care ultrasound provider, you are not trained in the proper technique to measure this. These images are here simply to illustrate the nuchal fold and how to generally “eyeball” it if a good sagittal view of the fetus is seen. But unless you have special certification to measure the NT or Nuchal translucency, do not attempt it. If it appears thickened to you, make the referral for a more advanced sonographic exam, and to have the mother counseled about possible genetic testing.

UTERUS AND ADNEXA

In addition to the gestational contents in the uterus, other uterine and adnexal structures are considered critical components, including:

--Uterine myometrial irregularities

--Ovaries

--Fluid in the cul-de-sac

--Pelvic masses or cystic areas

--Corpus Luteum cyst: Common

1) Can be enlarged or hemorrhagic, usually less than 10-cm

2) May have thick walls

3) Typically resolves by 16 wks

Figure 19. First trimester: Uterus and adnexa.

Sometimes the fetus will be laying in what appears to be on its side, making it more difficult to measure a crown rump length in that view. Remember that “up” on the screen represents what is facing toward the transducer. So although it seems that fetus is looking at you, it is actually looking sideways. If you are using a transvaginal probe, you may not be able to maneuver the probe to a location that will give you the” profile” view (fetus looking up). Then it is acceptable to measure the fetus from head to rump in this position, although not ideal.

Figure 20. Illustration of how the ultrasound wave transsects the fetus, and resulting image. Image 34.

Although a first trimester POCUS (crown rump length and viability) scan is not a “standard” exam, it is still important to survey the adnexa. When possible, even with a point-of-care ultrasound, try to get images and measurements of both ovaries, and be sure to view the cul-de-sac. It is also important to scan the entire uterus to look for uterine abnormalities like fibroids.

Once the uterus enlarges (in the second trimester), it becomes more difficult to view the adnexa from around the uterus, and a transabdominal approach is more helpful.

Images 35, 36. First trimester IUP and right ovary.

Below is a transabdominal view, but the fetus is now larger, allowing us to see limbs at 10wks 4 days. At this point in pregnancy, it can become more challenging to get an accurate view of the baby in the position that gives us the ideal measurement of the crown-rump length.

Image 37. Transabdominal image of a 10w4d fetus.

As the fetus grows and becomes more active, it is often difficult to view the fetus in the ideal “neutral”, “kidney bean shape” axis. This can skew the crown rump length if the fetal trunk is extended, making the measurement less accurate.

The fetus in image 38 below is stretched out, and the measurement may not be as accurate as if it had been curved in the usual kidney-bean shape.

Image 38. Fetus at 10w6d demonstrating extension of the body.

Transabdominal vs Transvaginal

Image 39 shows a 10 wk 6 day fetus using the transabdominal ultrasound. It is noticeable that the image clarity is not as good as with a transvaginal probe. However, at this stage of pregnancy you can still get a good measurement of crown-rump length. Usually, the transabdominal ultrasound is adequate to use at around 8-9 weeks. A good image can be seen, provided that the uterus is anteverted or anteflexed, there is a relatively full bladder, and the woman does not have a high BMI.

Image 40 on the right (shown for comparison) is the same gestation, except for using a transvaginal probe

Images 39,40. Comparison of transabdominal view versus transvaginal view of fetus of the same gestation, demonstrating increased detail with the TVUS.

Fetus at 12-13 weeks

At the end of the first trimester and beginning of the second, there are differences you will note in the fetus. As this image shows, bony structures are now becoming evident. This slide is also an example of how to be certain you have the longest view of the fetus possible when you are measuring. It is easy to mistake the image on the right to be a long view, when in fact it is a transverse or cross-section of the fetus. This could alter your measurement and gestational age calculation.¹.

Images 41, 42. Fetus at 12-13 weeks, viewed at 2 angles.

First Trimester Exam

At this gestation more detail of the limbs is visible – hands and feet. The developing spine is also easily seen.

Image 43. Image of fetus showing hand, legs, and spine.

At this point you can also begin to make out facial features.

Image 44. Fetus at 12 wks showing early facial features.

*Video of fetal movement

It is also good to document that fetal movement was seen. It would be beneficial If you have the ability to record and save it to your record or archives,.

To view this video clip, right-click on the image and you will see a tab below that allows you to play it.

Biometric measurements of other than CRL

Toward the end of first trimester, fetal flexion and extension can limit accuracy of crown-rump length measurement. If the ultrasound displays “OOR” (“Out of Range”) on the screen when trying to measure a CRL that is too long, then other biometric measurements become necessary.

The biparietal diameter (BPD) then becomes the preferred biometric measurement.

It must be measured in a transverse section at the level of the thalami. Accuracy of BPD between 12-16 wks is plus or minus 3-4 days.

Biometric fetal measurements are explained in detail in the Second/Third Trimester Course.

Figure 21. Biometric measurements used in lieu of CRL in late first trimester. Image 45.

It is also possible to measure the femur, although it is very small.

Image 45. First trimester femur length.

Other Structures: Placenta

At about 8-10 weeks the placenta can often be seen as an area slightly brighter than the myometrium, and extending inwardly from the chorion.

Image 46. First trimester placental tissue.

It is common to see the placenta appear near or over the cervix in the first and early second trimester. DO NOT CALL THIS A LOW-LYING PLACENTA AT THIS GESTATION. The placenta appears large in comparison to the relatively small uterus, so it can appear to be occupying a space near the cervix. As the uterus expands during the pregnancy, the placenta will typically be seen higher on the uterine wall; in fact, 85-95% of placentas, that initially appear to be low, resolve by the second/third trimester. Low-lying placenta will be discussed in detail in the Second/Third Trimester Course.

Image 47. Late first trimester image showing anterior placental edge at the internal cervical os. This demonstrates common position of the placenta at this gestation and does not predict a future low-lying placenta.

Uterine Contraction
You may also commonly see a “bunching” of the uterine wall toward the sac and the fetus. This is NOT an abnormality, but simply a uterine contraction. The difference between a contraction and a fibroid, is that there is not a distinct border on the contraction. Another difference is that a contraction will bulge inward, and a fibroid is more likely to bulge outwardly, distorting the uterine contour. The way you can tell for certain is to wait and rescan in about 10-15 minutes, and usually the contraction disappears.

Image 48. Uterine bulge representing a uterine contraction, a common transient finding.

Images 49 and 50 are two views of a uterine fibroid. It is noticeable that the fibroid has more distinct borders and mixed ehogenicity internally. This appearance is different from the uterine contraction.

Images 49,50. Two views of a posterior uterine fibroid in early gestation. Borders differentiate this from a uterine contraction.

Cervix in early pregnancy

It is good to view the cervix in first trimester, although it is not necessary to measure its length. In this long view of the uterus, the cervix appears to be long and closed. If a woman is bleeding or cramping, seeing the cervix closed like this with no hypoechoic regions to indicate blood or fluid in the endocervical canal, can be reassuring. It is during the 14-16 week gestational timeframe that cervical length needs to be measured in women who have had a prior preterm labor, cervical incompetence, or mid-trimester loss. Cervical length at that gestation is reviewed in the Second/Third Trimester course.

Image 51. Normal cervix, appears closed.

An example of an aberration in the anterior lower uterine segment/cervix is shown in Image 52. This particular irregularity was seen in a woman who had a prior C-Section, with a scar defect. The irregularity was followed and subsequently developed into a placenta accreta (described in detail in the Second/Third Trimester Course).

Image 52. Anterior lower uterine segment irregularity in what appears to be a C-Section scar defect.

Ovaries in early pregnancy

It is important to scan through the adnexa in early pregnancy to rule out a cyst or abnormality. As the uterus grows, the ovaries may not be seen as easily as in a GYN exam. A corpus luteum cyst in one of the ovaries is normal and offers reassurance that it is likely producing progesterone that helps maintain the pregnancy. However, it is possible to see a CL cyst in a non-viable pregnancy; so the presence of a CL does not always indicate an IUP or viability.

Image 53. Normal ovary in first trimester.

Discrepancy between dating of pregnancy by LMP and dating by clinical findings.

Establishing the estimated gestational age and due date are, as stated earlier, critical for decision-making later in the pregnancy. But when there is discrepancy between the LMP dating, the BhCG levels, and the ultrasound measurements, reconciliation of these is done by use of a standardized guideline established by the American College of Obstetricians and Gynecologists (ACOG). The following table lists those guidelines.

ACoG Methods for Dating pregnancy¹⁵

Table 8. ACOG (American College of Obstetricians and Gynecologists) guidelines for correcting a due date based on discrepancy between LMP dating and ultrasound measurements¹⁵

An example of using this table is as follows:

Using an LMP date of a woman her estmated gestational age is 7w 3d. A transvaginal ultraound measurement of the CRL gives an estimated gestational date of 8w 6d. Since this discrepancy is greater than 5 days (using the above table), the CRL date is used as being more accurate than the LMP date.

The next 3 chapters in this course describe the most common complications of early pregnancy that can be evaluated by a clinician using POCUS.

Sonographic signs suggesting an abnormal pregnancy in early 1^st trimester:¹⁶

1. Sac too large and/or empty

2. Sac too small for contents

3. Poor decidual reaction (<2mm)

4. Weak decidual echo amplitude

5. Low implantation

6. Irregular gestational sac/yolk sac shape

7. Duplication or absence of yolk sac

SUMMARY

The most critical indications for performing first trimester ultrasound are:

Establishing location, dating, and viability of the pregnancy

Ruling out ectopic pregnancy

Establishing an accurate due date; allowing for improved decision-making related to preterm or post-term management

CRL is more accurate than gestational sac measurement. Once the embryo is seen, only measure the CRL.

CRL should ideally be measured in the sagittal view of the embryo/fetus in a “neutral” position.

Pregnancy serum measures (b-HCG levels) alone should not be used to date a pregnancy, but these levels are used to correlate with ultrasound findings to determine discrepancies

3. Other indications for first trimester ultrasound

Look for uterine anomalies or masses (fibroids)

Observe the cervix – do not measure

View the at adnexa for abnormal structures (ovarian or adnexal masses)

SUMMARY

The most critical indications for performing first trimester ultrasound are:

1. Establishing location, dating, and viability of the pregnancy

2. Ruling out ectopic pregnancy

3. Establishing an accurate due date; allowing for improved decision-making related to preterm or post-term management

4. CRL is more accurate than gestational sac measurement. Once the embryo is seen, only measure the CRL.

5. CRL should ideally be measured in the sagittal view of the embryo/fetus in a “neutral” position.

6. Pregnancy serum measures (b-HCG levels) alone should not be used to date a pregnancy, but these levels are used to correlate with ultrasound findings to determine discrepancies

Other indications for first trimester ultrasound

1. Look for uterine anomalies or masses (fibroids)

2. Observe the cervix – do not measure

3. Look at adnexa for cysts

YOU MAY NOW TAKE THE QUIZ AND MOVE TO THE NEXT CHAPTER. YOU WILL NOT BE ABLE TO OPEN THE NEXT CHAPTER UNTIL YOU COMPLETE AND PASS THE QUIZ WITH 100%. YOU WILL HAVE AS MANY TRIES AS YOU NEED TO GET ALL THE ANSWERS CORRECT!

Women's Point-of-Care Ultrasound: First Trimester

First Trimester I Normal and Variations