Basic Obstetric POCUS

Chapter 7. First Trimester

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:

Identify pelvic structures with ultrasound in a non-pregnant woman

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

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.

First Trimester normal and variation slide

According to ISUOG (International Society of Ultrasound in Obstetrics and Gynecology) the main goal of performing first trimester ultrasound is to facilitate delivery of optimized antenatal care, ensuring the best possible outcomes for mother and fetus

There has been some controversy about the need for routine first trimester performed in normal pregnancy. If a woman is only allowed one ultrasound in the pregnancy, some feel that the second trimester anatomy scan is of more value than a first trimester exam. . However, the first trimester scan is probably the most important one in terms of confirming gestational age, called a “dating ultrasound”. Many critical management decisions throughout the rest of the pregnancy are based on accurate pregnancy dating. 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.

In some regions, women lack access to first trimester care, so gestational age would be confirmed later in pregnancy with reduced accuracy.

First Trimester Indications for POCUS

Figure 1. Indications for first trimester ultrasound

Indications for first-trimester sonography include, but are not limited to*:

Conducting routine assessment (screening) of low-risk pregnancy
Confirming presence of IUP (intrauterine pregnancy)
Estimation of gestational (menstrual) age
Determining viability by cardiac activity
Determining gestation number, and amnionicity/chorionicity of multiples
Evaluating a suspected ectopic pregnancy
Evaluating a suspected hydatidiform mole
Reconciling discrepant uterine size for gestational age
Determining causes of vaginal bleeding
Evaluating pelvic pain
Evaluating maternal pelvic masses and/or uterine abnormalities
Removal of IUD in early pregnancy

*American Institute of Ultrasound in Medicine (AIUM) and ISUOG

Comment

The first timester POCUS exam is considered a limited exam, even though it may have most of the components of a “complete” early pregnancy ultrasound. The POCUS scan would be done in response to the indications listed above.

First Trimester Exam Components

When a first trimester ultrasound is done, the structures to observe or measure (when they appear) include:

Gestational sac
Yolk sac and membranes
Embryo/Fetus
Crown-rump length
Fetal heart motion
Adnexa and uterus

Figure 2. Essential components of the first trimester exam

POCUS First Trimester

The first trimester POCUS is done solely to establish viability and measure a simple crown rump length. Although it is considered a “limited” exam, it still 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 follows: “A limited examination is performed when a specific question requires investigation.” AIUM also gives a list of other conditions to evaluate during pregnancy as part of a limited exam.

So the specific question in mind here is to determine pregnancy location and viability. 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. We will review early pregnancy loss and ectopic pregnancy in the next chapter.

In some areas women will have pregnancy lab results (such as urine or blood testing) prior to an ultrasound. When these results are available, they are a valuable resource to correlate with ultrasound findings in order to confirm gestational age or complications. If not, then accurate ultrasound evaluation becomes even more important.

Confirming Pregnancy

Why is confirmation of pregnancy in the first trimester Important?

--It establishes viability and location of the pregnancy

--It establishes gestational age.

Figure 3. Purpose of early confirmation of pregnancy

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 size discrepancy later in pregnancy would be better interpreted as an indication of a fetal growth abnormality rather than a different gestational age.

Figure 4. Indications for first trimester ultrasound

Critical Clinical Decisions

Why is early pregnancy measurement important?

--POCUS is the best method for estimating delivery date

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

Figure 5. Rationale for early pregnancy dating: impacting clinical care

Detecting and Confirming Pregnancy Urine testing

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

A URINE test is QUALITATIVE – meaning it is either positive or negative and does not give a numeric value. It is typically positive around the time of the first missed period.

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)

Serum pregnancy testing will not be reviewed since this is not available for the midwives in Gulu.

Figure 6. Use of urine testing in detecting and confirming pregnancy

Expected findings on Ultrasound by Menstrual age

If you use last menstrual period (LMP) as an indicator, and it is reliable (the woman has regular cycles preceding the pregnancy) then you can use this ‘menstrual age’ to help you determine what you should expect to see on an ultrasound. This is dependent on the reliability of recall of the woman, so it is not always accurate. This makes early ultrasound even more important in establishing an accurate due date.

Figure 7. Expected findings on ultrasound based on menstrual age of the pregnancy; comparison of using transabdominal versus transvaginal

At 6 weeks you should see a gestational sac.

At 6-7 weeks, you should see a yolk sac (YS) and the gestational sac (GS). 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 6-7 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, you can look at printed standardized tables that list 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 reviewed at the end of this chapter.

Figure 8. Components of a first trimester ultrasound exam, including POCUS

Gestational Sac

The first structure to be seen by ultrasound in early pregnancy is the gestational sac (GS). It is expected to be seen at 6 weeks, or when the sac is 2-5 mm in size.

Figure 9. Identifying a gestational sac

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 machines calculate this, and some calculate it with just 1 or 2 diameters rather than 3. Most ultrasound societies rely on 3 separate measurements of the GS diameter to arrive at an average. This is the sonographic standard known as the mean sac diameter. Diagnostic criteria for early pregnancy complications are based on this value, also known as the ‘MSD’. This is reviewed later in this chapter.

Scanning method

The preferred method for scanning in the first trimester is TRANSVAGINAL. However in the absence of availability of a TV probe, a transabdominal (TAUS) ultrasound is acceptable. However, structures will not be seen as early as they would be if using a transvaginal approach.

Although transabdominal ultrasound (TAUS) is acceptable, it does require a full bladder. Views of very early 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 gestational sac (GS).

Imaging Early Pregnancy Components

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 next chapter reviews this in greater detail.

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

Figure 10. Description of when to visualize a GS

The sonographic view of an early GS is seen in the images in Figure 11. These images compare the view from a transvaginal scan (left image) versus that from a transabdominal scan (right image).

Figure 11. Images demonstrating early GS

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.

Figure 12. Image showing a decidual reaction surrounding a GS.

Figure 13. Illustration of the anatomy of an early gestation

This diagram shows 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.

Mean sac diameter measurement

Measuring a gestational sac helps to confirm gestational age, or discover a discrepancy. When measuring a GS, it is important to measure it in 3 dimensions to get the “mean” measurement. This mean diameter is the most accurate in dating the pregnancy before an embryo can be seen.

Figure 14. Mean sac diameter measurement

This chart is a quick review of how to correctly measure a first trimester gestational sac.

This table defines precise measurement of a mean sac diameter.

In review, 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.

Figure 15. Image of gestational sac measurement

The image above (Figure 15) illustrates the use of 3 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 width after rotating the probe 90 degrees..

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.

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 6 weeks, using a transabdominal probe. An absent GS could indicate incorrect LMP dates, early failed pregnancy, or an ectopic pregnancy.

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.

Early pregnancy loss and ectopic pregnancy 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 (YS) and the amnion.

The yolk sac

--Expected at 6 wks by transabdominal scan

--Located outside the amnion but inside the chorion (decidual ring)

--Echogenic ring with anechoic center

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

--Size – large size (greater than 6 mm) 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. It surrounds the embryo or fetus, but does not completely fill the gestational sac.

--Separate from the chorion until 14-16 wks, when they are fused.

Embryonic Anatomy

Terminology - note that here and in other references you will see the term “fetal pole” and “embryonic pole” used interchangeably. They are the same thing. The technically correct term for a fetus prior to 7-8 weeks would be embryo and later than 8 weeks considered a fetus.

Figure 16 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 visible at 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).

When observing these structures, keep in mind their relationship to one another.

--The fetus is inside the amnion, which is inside the chorion.

--The yolk sac lies between the amnion and the chorion.

Figure 16. Illustration of embryonic anatomy

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 first trimester, when placental function takes over.

Figure 17. Illustration of relationship of GS, yolk sac, amnion, and embryo

There is a stalk, called the vitelline duct, that connects the fetus to the YS. It is the part 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. The YS will always be where the pregnancy is located.

Figure 18. Image of yolk sac attached at the end of the vitelline duct

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

Figure 19 is a graphic representation of the relationship of the yolk sac, amniotic sac, and chorion (which attaches to maternal side).

Figure 19. 3-D illustration of the embryo, YS, chorion, and amnion

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

This 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.

Figure 20. Image of early first trimester structures

The embryo first appears as a “grain of rice” attached to the YS. This is sometimes referred to as the “diamond ring” sign (Figure 21).

Figure 21. Early view of tiny embryo attached to the YS

A fetal pole is expected to be seen at 6 weeks’ gestation with TAUS, with some exceptions such as uterine retroflexion or large maternal body habitus.

Figure 22. Decidual ring around GS, early view of YS

The round ring in the image above (Figure 22) 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”. You can also see that there is a good decidual ring.

Figure 23 shows images of the embryonic pole contiguous with the YS. This is normal at this stage.

Figure 23. Images of early fetal pole (embryo) and YS

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

Figure 24. View of tiny embryonic pole

Figure 25. Image of embryonic pole measuring 7.0 weeks gestation

This is an image of a fetus at 7 weeks, and it demonstrates a good bit more definition and echogenicity.

This image (Figure 26) is 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.

Figure 26. Image of embryonic pole and 3-d illustration of “real” appearance

Figure 27. 3-D image of early embryo and YS

Figure 28. Illustration and image of relationship of fetal pole, amnion, and YS

Figure 28 depicts 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.

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:

8-14 mm at 6 wks

14-20 mm at 7 wks

21-30 mm at 8 wks

Figure 29. Description of measuring the early fetus

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

The chart below (Figure 30) is a quick review of how to correctly measure a first trimester embryo/fetus, defining precise measurement.

Figure 30. Parameters of measuring the embryo/fetus

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.

Embryo

Figure 31 demonstrates an early CRL of 6.5 weeks. Just below the fetal pole, you can see the thin wisp of the amnion.

Figure 31. Image of embryo and amnion at 6 wks and 5 days.

This image (Figure 32) at 7wks 0 days gestation shows a well-defined fetus, along with a vitelline duct connected to a well-defined yolk sac.

Figure 32. Image of embryo at 7 weeks

Below (Figure 33) is an image of a 7wk 1 day gestation. The image is very clear since it is used with a more sophisticated ultrasound machine having excellent resolution.

Figure 33. Embryo at 7wks 1 day, and adjacent YS

Figure 34 is an image of gestation 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).

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

Figure 34. Image of a fetal pole at 8wks 5 days

First trimester cardiac

After determining pregnancy location, the second most important thing to observe is viability. Cardiac activity begins at about 5 weeks from the LMP, and cardiac motion can be observed with ultrasound when the embryo is 5 mm or greater.

Figure 35. Fetal cardiac activity

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.

Figure 35. Cardiac contractions

M-mode is what is used to measure fetal heart rate in first trimester. M-mode allows us to document fetal heart rate 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.

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 36. Definition of 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.

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)

M-Mode is demonstrated in Figure 37 (below).

Figure 37. Image of application of M-Mode

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 37 is a very early 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.

Figure 38. Image of M-mode at 7 wks and 5 days

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 captured on an image, 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.

Figure 39. Other components to observe in late first trimester

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.

Rhombencephalon

Below are 2 images of rhombencephalon (Figure 40)

Figure 40. Image of rhombencephalon (normal finding)

Figure 41 (below) also demonstrates a bit of midgut herniation (sometimes called embryonic foregut), as well as a rhombencephalon. These are common normal findings.

Figure 41. Image of additional features of embryo.

Late First Trimester

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

Crown-rump Length

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

When the fetus grows, it can begin to flex and extend its torso, sometimes making the ideal view of the “kidney bean” shape difficult to see. The fetus in Figure 42 is stretched out, and the measurement may not be as accurate as if it had been curved in the usual kidney-bean shape. It will still be close.

Figure 42. Image of first trimester fetus with crown rump measurement

Figure 43. Images of a 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. Figure 43 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.

At 12 weeks more limb detail is visible – hands and feet. The developing spine is also easily seen (below in Figure 44).

Figure 44. Image of fetus at 12 weeks showing spine and CRL measurement

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

Figure 45. First trimester image of early placenta location and some fetal facial features

Video 1. First trimester 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.

Figure 46. Using biometric measurements other than CRL in late first trimester

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.

First Trimester Exam Femur Length

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

Figure 47. Image of first trimester femur length

First Trimester Pelvic Structures

In addition to measuring 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 48. First trimester uterine and adnexal components to observe

Placenta

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

Figure 49. Image of early placenta location

It is common to see the placenta appear near or over the cervix in the first and early second trimester (as shown in Figure 50 below). 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.

Figure 50. Image of anterior placenta that is approaching the internal os.

Uterine contractions

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.

Figure 51 shows an image of a uterine contraction. There are no clear borders on this structure, indicating that it is likely a contraction. Figure 52, on the other hand (below), has definite hypoechoic borders indicating that it is likely to be a fibroid.

Figure 51. Image of uterine contraction during first trimester exam

Figure 52. Image of uterine fibroid (demonstrating borders)

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 chapter on maternal-fetal structures.

Figure 53. Image of the cervix in first trimester (fundus is to the left and cervix bows upward and toward the right)

Ovaries in early pregnancy

When possible 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.

Figure 54. Transabdominal images showing ovaries adjacent to the pregnant uterus

Viewing the adnexa in early pregnancy can rule out a cyst or other pelvic 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.

Once the uterus enlarges (in the second trimester), it becomes more difficult to view the adnexa from around the uterus.

Figure 55. Image of ovary with a CL in first trimester

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.

•Other indications for first trimester ultrasound:

Look for uterine anomalies or masses (fibroids)
Observe the cervix – do not measure
Look at adnexa for cysts

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

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. YOU WILL HAVE AS MANY TRIES AS YOU NEED TO GET ALL THE ANSWERS CORRECT!

GULU Midwives Basic Obstetric POCUS

Chapter 7. First Trimester