Basic Obstetric POCUS

Chapter 9. Maternal-Fetal Structures: Placenta, Cervix, Cord

Welcome to the chapter on ultrasound of the placenta, cervix and umbilical cord. This chapter will review placental location and characteristics, including identification of the low-lying placenta and placenta previa. In addition, the classification of placenta accreta conditions known as PAS or Placental Accreta Spectrum is presented.

The middle section of this chapter discusses cervical evaluation. Included in this review is cervical length measurement and cervical changes that could predict preterm labor.

The last section discusses basic evauation of the umbilical cord. Although evaluation of the umbilical cord is typically a part of the standard 20-week fetal anatomy scan, it is prudent for the POCUS provider to look at the location and characteristics of the cord.

Objectives

By the completion of this chapter the participant will be able to:

Correctly identify posterior, anterior, fundal, and low-lying placentas
Describe the categories of placenta previa
Demonstrate how to measure distance from leading edge of placenta to the internal cervical os
Describe vasa previa and how to identify it
Differentiate between uterine fibroid and uterine contraction
Describe the types of placentation in twin gestation
Describe the categories of placenta accreta spectrum (PAS)
Describe how to measure cervical length and identify cervical funneling
Describe characteristics of the umbilical cord that can be evaluated with POCUS

PLACENTA

Placenta facts

It is well known that the placenta is the most important organ for fetal survival since it is the organ that transfers nutrients and oxygen from the mother. Evaluation of the placenta helps to identify potential problems that could affect fetal growth and health.

Figure 1. Placenta facts

Some interesting facts about the placenta are:

Average weight: approximately 500-600 grams at term,
Normal placental size: 15-20 cm in diameter and 2-3 cm in thickness at term,
The placenta contains approximately 50-60 cotyledons, each with 1-5 lobules.
Blood flow to placenta (critical for fetal survival) is approximately 500-600 cc per minute

The primary reason for evaluating the placenta is to evaluate its function of sustaining the fetus. If there are abnormalities in structure or implantation, the fetus can suffer damage

Figure 2. Reasons for evaluating the placenta

Abnormalities of placenta contributing to a number of pregnancy complications include:

Increased incidence of fetal and/or maternal morbidity and mortality:
Hemorrhage
Retained placenta
Decreased fetal growth (described in fetal growth module)
Increased association with Preeclampsia and Gestational Hypertension
Increased incidence of operative delivery
Retained placental tissue after delivery due to failing to detect a succenturiate lobe or other abnormality prior to birth

Optimal image of the placenta

Several techniques are recommended for effective ultrasound viewing of the placenta:

Use highest frequency possible for best resolution
Use the “Routine 2^nd trimester” preset
Adjust depth, gain, focal zone, or zoom as needed to enhance the image
Image with and without color doppler to observe vascularity
Scan entire placenta/uterus in sagittal and transverse views (to identify possible succenturiate lobes)

Figure 3. Optimal placental image

Viewing the placenta with color flow provides an overview of its circulation. In the second and third trimesters, use of color Doppler does not pose a risk to the fetus, as it may in first trimester. You should see evenly distributed vascularity throughout the placenta.

Figure 4. Image of placenta using color Doppler to evaluate placental circulation

ISUOG (Internatinoal Society of Ultrasound in Obstetrics and Gynecology) recommends assessing the placenta for major abnormalities as early as 9 weeks' gestation, which requires a high-resolution machine and TVUS.

ISUOG recommends evaluating placental size and texture as early as between 11-14 weeks with transabdominal ultrasound.

Figure 5. ISOG guidelines for imaging the placenta in First Trimester

The components of the second and third trimester placental evaluation are:

Observing Placental Location
Relationship to the cervical os; measuring the distance between the lowest edge of the placenta to the internal cervical os.
Appearance (any structural abnormalities or extra lobes)
Placental cord insertion (identifying marginal cord insertion, which can sometimes be associated with fetal growth restriction)
Abnormal adherence, especially with a history of uterine surgery like C/Section

Figure 6. Essential components of placental evaluation

Knowing the placenta’s location helps identify risks like low placement and abnormal bleeding, and aids management if a C-section is needed.

Location is described as anterior (oriented toward the maternal abdominal wall and the transducer) or posterior (positioned towards the lower portion of the ultrasound screen, corresponding to the woman’s spine and further away from the transducer). In a transverse view, it is possible to identify whether placenta lies more toward the maternal left or right side. Additionally, in a sagittal plane, one can determine if the predominant portion of the placenta is situated on the left side of the screen (toward the uterine fundus), or on the right of the screen - near the cervix.

If the placenta appears to be at the top of the uterus, it is called a fundal placenta. Portions of the placenta may appear to extend anteriorly and posteriorly.

Figure 7. Localization of placenta

The image below demonstrates an anterior placental location (toward the anterior uterine wall)

Figure 8. Image of an anterior placenta

This next image (Figure 9) demonstrates a posterior placenta (toward the maternal spine)

Figure 9. Annotated mage of a posterior placenta

When looking at the placenta in the sagittal view to the mother, we can determine whether it is anterior or posterior. But to know if the placenta is more toward the left or right of the uterus, we need to scan the uterus in a transverse plane. In Figure 10 (below) we see that the placenta is all along the anterior wall of the uterus, but the larger portion of it is toward the right side of the uterus.

Figure 10. Image of a placenta viewed in a transverse plane to the uterus

The placenta in Figure 11 (below) is anterior. Note that the placenta will appear anterior whether viewing it sagittaly or transversely since it lies closest to the maternal skin line and the transducerk in either plane.

Figure 11. Placenta- orientation

Figure 12. Same image as Figure 13 with red line outlining

Figure 13 (below) is sagittal image of a placenta that lies posteriorly, but with a part of it curling up toward the fundus (the left side of the image).(also note face of baby). This means that baby’s head is near the fundus, and the face is likely pointing to the mother’s right side, making it look like the baby is looking at us.

In this sagittal view, the baby’s head is positioned near the fundus and the placenta lies posteriorly, with part curling up toward the fundus (left side of the image).Figure 13. Image of posterior placenta.

Figure 14 (below) presents a transverse view of the abdomen, indicating that the placenta is located anteriorly, with the majority positioned towards the maternal left. The most appropriate description for this placenta would be anterior/left .

Figure 14. Image of a placenta taken in the transverse plane of the mother

Below (Figure 15) is a sagittal view of the mother, demonstrating that the placenta is at the fundus. Some of it wraps upward to be anterior, but most of it lies on the posterior side of the uterus. We would describe this as a posterior/fundal placenta.

Figure 15. Image of a fundal placenta

The left side of the screen in the image below (Figure 16) corresponds to either the mother's head or her right side. In a sagittal view of the mother's abdomen, the baby's head is positioned toward her head, and the placenta is located anteriorly. The position of the head at the fundus indicates the fetal position as breech since we know the head is not at or in the pelvis.

Figure 16. Sagittal view of anterior placenta

Placental size

The size and shape of the placenta relates directly to uteroplacental circulation, so it is important to note these when viewing the placenta.

Conditions associated with large placenta

Diabetes
Rh Sensitization
Viral infections
Syphilis
Some chromosomal abnormalities such as Triploidy

Conditions associated with small placenta

Intrauterine growth restriction
Abnormal placentation
Preclampsia
Severe insulin-dependent diabetes
Chronic infection (including COVID)
Chromosomal abnormalities

Placental thickness ranges from around 1 cm at 10 weeks gestation, to 2 cm at 20 weeks, and 3.25 by term. Thickness and size can be related to inflammatory processes affecting placental/fetal circulation, or from maternal conditions such as infection, diabetes and hypertension. We do not expect you to make a measurement of the placenta as part of your 2nd and 3rd trimester placenta observation, but to know these parameters to apply if a placenta seems small or thin.

Placental grading

Previously, sonographers assessed placental maturity by examining its fetal surface and echogenicity, using changes in texture as indicators aligned with gestational age. Presence of numerous calcifications can indicate an “aging” placenta. Use of this grading system helped detect early aging and possible insufficiency in high-risk pregnancies. Although the grading system is being used less in practice, if a Grade III placenta (marked by pronounced indentations and calcifications) is observed before 36–38 weeks, it is significant and warrants consultation. Some researchers maintain that placental grading remains relevant to predicting pregnancy outcomes. Your local obstetric providers and sonographers may still use this system, so it is good to understand and use the correct terminology.

Figure 17. Placental grading and sonographic characteristics

Placental hematomas are sometimes seen. The difference between a placental hematoma and a small abruption may be difficult to determine. But knowing what they look like will help you to identify those that need additional expert imaging. Note that the hematoma is located between the placenta and the uterine wall, and not in the placenta.

Figure 18. describes various types and locations of placental hematomas. In early pregnancy we call it a subchorionic hematoma, as described in the chapter on first trimester. In some cases these hematomas persists into later gestation, although many will resolve.

Figure 18. Illustrations and images of placental hematomas

Placental lakes (called lacunae) are typically benign if they are small. They are identified as dark sonolucent areas within the placenta or on the periphery. These are very common and are usually not of any clinical significance. These are typically in the placenta or on the fetal side of the placenta, so they are not hemorrhages. If you apply color flow, you will not see any motion, because these are static pools of blood. But sometimes small amount of “sloshing” of the blood can be detected by the more sensitive power Doppler.

Figure 19. Placental lakes

The number of lakes does not appear to be problematic, but the size may represent a problem if it exceeds 5 cm. There may also be concern venous lakes appear in early second trimester.

Figure 20. Image showing venous lakes

LOW LYING PLACENTA

Discussion will now focus on low-lying placenta and placenta previa. The most common cause of maternal death related to childbearing is maternal hemorrhage. One of the most common causes of hemorrhage is due to a low-lying placenta or previa separating from the uterus prematurely as the cervix begins to dilate in labor. If hemorrhage begins during labor and the cervix is not fully dilated, the fetus can also succumb due to significant maternal bleeding.

Ultrasound is one of the best ways to determine if the placenta is near the cervix, and can be instrumental in saving lives if preventive measures are taken with these women.

A low-lying placenta is defined as a placental edge located within 2.0 cm of the internal cervical os (internal os distance, IOD). Accurate measurement is achieved using transvaginal or translabial ultrasound with an empty bladder. Given that you do not have access to a transvaginal probe, it is still important for you to survey the proximity of the placental edge to the internal cervical os. If measurements seem abnormal, referral for a transvaginal ultrasound should be done, especially if the woman is near term or having bleeding.

Figure 21. Definition of low-lying placenta

Figure 22. Considerations for scanning a low-lying placenta

Measuring placental edge to internal cervical os

The method for viewing the internal os distance (IOD) is similar to that used for measuring cervical length. As with cervical length measurement, applying excessive pressure may distort the lower uterine segment and affect the accuracy of the measurement. It is also important to view the distance at several different angles in the suprapubic region. The reason for this is that if your probe is directly midline, the placental edge may appear to be far enough away from the cervical os. But if the low-lying portion of the placenta is either to the left or right, that portion may be closer to the os. So it is good to sweep your probe to the right and left suprapubically.

Figure 23 (below) shows a red arrow marking the lowest edge of the posterior placenta, and a blue arrow indicating the indentation at the internal cervical os.

Figure 23. Sagittal view of the uterus identifying the internal cervical os and the posterior placental edge

To determine if this is a low-lying placenta, the distance between the specified points is measured at 2.5 cm shown in Figure 24 below. The established criterion for a low-lying placenta is 2.0 cm or less. While this placenta is positioned lower, it does not meet the definition of low-lying based on these criteria.

Figure 24. Same image as in Figure 23, demonstrating how to measure from the placental edge to the internal cervical os

Below is an image of an anterior edge is positioned at the margin of the internal os. This would be considered a low-lying placenta. HOWEVER, at this gestation it very likely that the placenta will move away from the cervix as pregnancy advances, meaning that there would be less risk. At this gestation we would make note that it appears low-lying and that follow up ultrasound later in second or third trimester is indicated to confirm it is no longer low-lying.

Figure 25. Low-lying placenta in early second trimester

The red lines in Figure 26 (below) demonstrate the edge of the placenta encroaching on the internal cervical os.

Figure 26. Same image as in Figure 25 with annotation in red

Figure 27. Posterior placenta near posterior cervical os (white arrow points to the lowest edge of the placenta)

The image in Figure 27 (above) demonstrates a low-lying posterior placenta with a measured distance of 2.2 cm, which does not meet the technical criteria for low-lying. However, there appears to be some cervical funneling, indicating that a transvaginal measurement would provide greater accuracy, as this image was obtained via a transabdominal approach.

The red dotted line in this image demonstrates the lower edge of this placenta and its proximity to the internal cervical os

Figure 28. Same image as Figure 27, placenta outlined in red

You may have heard of the term “migration” pertaining to placentas “moving” away from the cervix as the pregnancy advances. It had once been thought that as the uterus enlarges, the placenta “moves” upward with it. It has recently been postulated by radiology experts that the lower edge of a low placenta does not have enough blood supply in the region of the lower uterine segment to support continued growth of the placenta, and the lower part of the placenta deteriorates because of this. Either way, we know that placentas often improved as the pregnancy progresses.

Figure 29. Do placentas “migrate”?

Most low-lying placentas or placenta previa resolve before term, with studies showing about 92% resolution within a median of 10 weeks from diagnosis. Even when the placenta is extremely low (within 0–10 mm), 95% resolve by term, and 72.3% of placenta previa cases resolve in roughly 12 weeks.

Figure 30. Facts about the low-positioned placeta

A 2020 study by Jansen et al reported that anterior low-lying placentas are more likely to resolve by term than posterior placentas, due to greater uterine wall expansion. If an anterior placenta is over 0.5 cm from the internal os, a third-trimester ultrasound may not be needed. In contrast, posterior placentas under 2.0 cm have a lower chance of migrating and should be re-measured later. Women with a low-lying anterior placenta and prior C-section should also be evaluated for placenta accreta syndrome by a skilled sonographer or physician.

Figure 31. Diagram of low-lying placenta.

Figure 32 Image of low-lying posterior placeta

Figure 33. Same image as Figure 32, with annotation; the red line marks the placental edge and its proximity to the internal cervical os

The image below shows an anterior, low-lying placenta via transabdominal scan, which is less accurate than a transvaginal approach. The presence of urine in the bladder and a possible anterior contraction further limit interpretation.

Figure 34. Low lying anterior placenta

Figure 35. Same image as Figure 34 but with annotation outlining the cervix and anterior placental edge in red

PLACENTA PREVIA

Placenta previa occurs when placental tissue covers all or part of the cervical opening, sometimes causing painless vaginal bleeding. It affects about 1 in 200–250 births.

Approximately 20 percent of these are complete previas, meaning that part of the placenta completely covers the os.

About 80% are marginal, which means that the os is only partly covered by placental tissue

The incidence of placenta previa at this time is as high as 42%. HOWEVER, 90% identified at 15-19 wks will resolve. The reason that the placenta appears low in early gestation is because the placenta grows rapidly in a relatively small cavity, appearing to occupy a large area.

As the uterus grows, the proportion of the placenta to the uterine cavity becomes smaller. Since there is a high rate of the placenta appearing to cover the cervical os at that time, it is important NOT to diagnose placenta previa at this stage – the vast majority of these will resolve within a few months.

Figure 36. Definition and incidence of placenta previa

Special attention is given to women with a history of prior C/Section, since an anterior placenta may decidualize anteriorly through the lower uterine segment or site of previous C/Section scar.

Figure 37. Factors that can mimic placenta previa

Placenta previa should be re-evaluated near term to assess if the placenta is within 2 cm of the os. Most patients can attempt vaginal delivery, but intrapartum hemorrhage risk is higher. If the placental edge is less than 1 cm, trial of labor is an option; if ≤1 cm, consider C-section or supervised trial of labor with anesthesia and blood products available. Delivery is indicated at 37–38 weeks if risk factors are present. With no other risk factors, delivery can be postponed to 38-39 wks.

Another commonly seen feature to note while observing the placenta is the presence of focal uterine contractions (as shown below in Figure 38). Distinguishing between placental and uterine tissue can be challenging, particularly in early pregnancy. Uterine muscle contractions may distort the cavity, making it hard for sonographers to tell if a bulge is due to a contraction or a fibroid.

A very full bladder can also distort the appearance of the placental relationship to the uterus and cervix.

Figure 38. Image of an anterior placenta lying over a uterine contraction (bulging toward the fetus)

PLACENTAL ABRUPTION

Placental abruption occurs when all or part of the placenta detaches from the uterine wall, which can range from mild to severe and may cause fetal hemorrhage or death. Warning signs of acute abruption include vaginal bleeding, reduced fetal movement, or sudden increased abdominal or uterine tension. If suspected clinically, immediate assessment by a qualified sonographer or physician is crucial; do not delay care trying to diagnose it with a POCUS.

Risk factors for placental abruption:

Hypertension
Smoking
Drug/cocaine use
Trauma
Short umbilical cord
Submucosal fibroids
Uterine anomalies
Placenta previa
Premature rupture of membranes

In an asymptomatic woman, a small separation of the placental edge may be seen on a routine scan, and would appear as a mass or collection of fluid between the placenta and uterine wall. Often the placental edge is lifted away (or detached) from the uterine wall at the edge, as shown in Figure 39 below .

Figure 39. Image of a placental abruption

Figure 40. Same image as Figure 39, with annotation

Placental abruption may occur with retroplacental fibroid(s), uterine contractions, or placental calcifications. If the area is small, the fetus is healthy and there are no maternal symptoms, follow-up ultrasounds are recommended.

Marginal placental abruption often occurs at the periphery and can be chronic rather than acute. Maternal risk factors include hypertension, smoking, and illicit drug use.

Figure 39 (above) demonstrates placental separation and relative hypoechogenicity between the placenta and uterine wall. Figure 40 highlights this with arrows. The blue arrow indicates the placenta. The mixed echogenic area represents a blood clot located behind the placenta. Placental abruptions can be challenging to identify using ultrasound, according to many radiologists. Retroplacental hemorrage can appear either hypoechoic, or hyperechoic, depending on the clotting stage of the blood. Therefore, these cases are often managed based on clinical presentation.

NOTE: It is difficult to detect a true placental abruption with ultrasound unless you are highly skilled. Acute abruption is an emergency condition that requires immediate action to prevent fetal loss, so it must be addressed rapidly. There is typically little time to look with ultrasound; the condition must be ascertained by clinical symptoms with rapid intervention.

PLACENTAL VARIATIONS

Several placental variations exist, including velamentous insertion, succenturiate lobe, circumvallate, battledore (marginal insertion), and bilobed. Bilobed placentas have two similarly sized areas, unlike succenturiate lobes. We do not expect POCUS providers to learn to detect these, but it is important to know the distinctions. Those who deliver babies know that abnormally configured placentas and cord configurations can lead to third stage labor complications, such as cord avulsion.

Figure 41. Other placental variations

The diagram in Figure 42 (below) shows a succenturiate lobe and highlights how it can be overlooked without close inspection of the uterus. A succenturiate lobe increases the risk of vasa previa (especially if the placenta is low) and can lead to postpartum hemorrhage if an extra lobe remains undetected after delivery.

Figure 42. Illustration and image of succenturiate lobe detection

Twin Placentation

As we described in the section on diagnosing twins, we can determine chorionicity if we see two placentas (and a lambda sign between). It is more difficult later in pregnancy to determine whether twins are diamniotic since the thin membrane separating them is often difficult to locate. This reiterates the importance of determining chorionicity and amnionicity as early as possible in the first trimester

Figure 43. Placentation variations of twins

Figure 44. Image demonstrating the lambda sign found in dichorionic diamniotic twins

In the pregnancy image above (Figure 44), the lambda sign and the thickened chorion separating the two amniotic sacs area clearly seen.

PLACENTA ACCRETA SPECTRUM

Placenta Accreta Spectrum (PAS), previously called Morbidly Adherent Placenta (MAP), has seen rates quadruple since 1980 according to the Society of Maternal Fetal Medicine (SMFM), with a current incidence of 1 in 272 births.

Figure 45. Facts about Placenta Accreta Spectrum (PAS) disorder

A primary risk factor is a history of having a C/Section. In addition to this, having an anterior and low-lying placenta can portend a risk.

Figure 46. Illustration of placenta accreta

This occurs because decidualization happens within a uterine scar, usually from a C-section. PAS increases maternal risk. Delivery should take place at a level III or higher facility with access to blood transfusion.

Figure 47. Illustration of placenta increta

The diagram in Figure 47 (above) illustrates that the normal placenta undergoes decidualization by invading the stratum basalis of the endometrium. In cases of accreta, this invasion extends beyond the stratum basalis into the myometrium.

Figure 48. Illustration of placenta percreta

Placenta percreta occurs when the placenta penetrates through the myometrium into the uterine serosa, extending to tissue beyond (sometimes reaching the bladder). This serious condition often leads to C-Hyst (cesarean delivery and hysterectomy) due to uterine damage or hemorrhage and typically requires a urologic surgeon for bladder repair.

Not all PAS is diagnosed ; some milder cases are not detected by ultrasound, but can nonetheless be serious.

Any woman with a history of prior C/Section and an anterior low-lying placenta should be evaluated by a sonographer or specialist trained to look for signs of PAS prior to labor. Often an MRI is an additional step in making the diagnosis.

All women with suspected PAS must be referred for high risk management.

If either placenta accreta, increta or percreta remain undetected, it can lead to significant postpartum hemorrhage and significantly higher morbidity and mortality, especially if the woman delivers in a setting without access to immediate surgical intervention. At the very least, most women with a significant placenta accreta will require a D&C to remove the adherent placenta after delivery.

CERVIX

This section of the chapter will outline the significance of cervical measurement in predicting preterm labor. It will also detail the proper techniques for measuring cervical length and provide guidance on interpreting these measurements for clinical management.

By the completion of this Section the participant will be able to:

1. Demonstrate the correct technique for measuring a cervix

2. Describe rationale for using transvaginal vs transperineal approach

3. Cite the normal cervical measurement parameters for less than 22 weeks, and 22-32 weeks

4. Describe degrees of cervical funneling and implications for preterm labor

Cervical effacement has traditionally been assessed by digital examination, but this method can be unreliable and may not reflect the true cervical length. It had been previously thought that the cervix thins and dilates from outer to inner. However, the opposite occurs in preterm labor; the cervix thins and dilates from the inside out, rendering a digital exam to be inadequate. In cases of premature rupture of membranes or active contractions, digital exams may not be advisable. Ultrasound provides a more accurate assessment.

Figure 49. Reasons for performing cervical length with POCUS

Measuring cervical length helps assess preterm labor risk. Women with a history of preterm birth are up to twice as likely to experience it again. Current guidelines recommend cervical measurement at around 15-16 weeks for those with a history of preterm labor. There are currently no recommendations to measure the cervix in women in early pregnancy without preterm birth risk factors.

Figure 50. Cervical length facts

Berghella’s Cochrane review shows cervical length measurement effectively predicts and helps prevent preterm birth in both symptomatic and asymptomatic women. It can also confirm or rule out incompetent cervix, assess the likelihood of imminent delivery in women with labor symptoms, and, according to ACOG (American College of Obstetricians and Gynecologists), identify those who may not require intervention.

Figure 51. Additional clinical situations for measuring cervical length

Measuring the cervix may also help:

· Predict success of labor induction (part of the Bishop score)

· Assess risk of pregnancy extending past term

· Guide timing for maternal transport in complication referrals

Cervical length measurement should generally not be performed prior to 14 weeks of gestation, as the uterine body and cervix are not distinctly differentiated before this period. For women at risk of preterm labor, cervical screening is recommended between 16 and 24 weeks. Screening beyond 24 weeks in asymptomatic patients has limited clinical utility and does not appear to improve patient outcomes. It is considered an optional procedure by SMFM and ACOG universal transvaginal cervical length screening in low risk patients should not be mandated.

Figure 52. When to measure cervical length

Figure 53. Cervical length measurement guidelines

As pointed out in Figure 53 (above) it is advised to take 3 measurements and record the shortest value. This practice accounts for cases where the cervix is considered dynamic, meaning its length may vary during an examination due to internal pressure or contractions.

Cervical lengths to remember:

4.0 cm is the average length prior to 22 wks.

3.5 cm is the average length from 22-32 wks.

After 32 wks., cervical length is variable, and is not necessarily predictive of labor. Also, after 32 weeks, management would not include tocolysis or trying to stop labor.

A critical measurement to know is 2.5 cm – which represents the 10^th percentile of cervical length. Anything below this value constitutes cervical shortening.

If cervical shortening is detected, consultation or referral for further assessment is imperative. Prior recommendations included use of supplemental progesterone with a cervical measurement of 1.0 to 2.0 cm. The next slide presents the ACOG clinical management pathway for a short cervix.

Evidence shows cervical length screening is not useful for predicting preterm labor in multiple gestation, PPROM, or placenta previa. Although CL measurement in twin gestation does not reduce the risk of preterm birth, it does have advantages. It can improve timing of antenatal corticosteroid treatment for those who will deliver preterm. While following good measurement principles is important, specific certification in cervical length imaging is not required.

Identifying and imaging the cervix

To identify the internal os, find a small dip or wing-shaped indentation. The external os is typically just inside the outer edge of the cervix, not at its very tip. Once the cervix is visible in the long plane, zoom in so it fills 50-75% of the image for clear viewing. Identify the cervical mucosa, which may appear hyperechoic or hypoechoic. (INSERT REVISED IMAGE SHOWING WING)

Figure 54. Illlustration and image of identifying the cervix for measurement

(insert new image)

Figure 55. Images demonstrating cervical length measurement

The cervix is more often curved than straight, so measuring its length can sometimes be challenging. The ultrasound machine may give you the option of measuring in a straight line or combination of 2 segment measures (Figure 56). Or use of a “trace” feature allows you to trace the line following the internal curvature of the cervix (Figure 57). The trace feature is more accurate, but harder to perform.

Figure 56. How to measure a curved cervix - adding 2 segments together for a total length (REPLACE)

Figure 57. Measuring a cervix using the “trace” feature (REPLACE)

Measuring a cervix with a full bladder can result in elongation or stretching due to bladder pressure, as seen in Figure 58 below. Typically, a normal cervix measures no more than 4 cm.

Figure 58. Cervical elongation with full bladder

The images in Figure 59 below show different transabdominal views of the cervix. If a transvaginal probe is unavailable, an estimate of cervical length can be obtained; however, this measurement is less accurate than with a transvaginal scan. A full bladder improves visualization of the cervix due to acoustic enhancement, but it may also falsely increase the apparent cervical length, which can result in a misleading measurement.

Figure 59. Images as examples of cervical measurement

Figure 60. Images comparing cervical measurement with a full maternal bladder vs an emptied bladder

The transabdominal image below (Figure 61) demonstrates cervical measurement at term. The cervix exhibits a hyperechoic os (brighter), likely indicating the presence of a mucus plug. It is important to note that bladder distension may be contributing to an elongated appearance of the cervix on this examination.

Figure 61. Image of a term cerix enhanced by viewing through a full bladder

Figure 62 (below) outlines MANAGEMENT for short cervix. Progesterone is still recommended for those with short CL, but not to be used prophylactically or those with a normal CL, even for those with a history of preterm birth. It is unlikely that you would be initiating this in your clinical practice, but be aware that in some places this is an option for preterm prevention management.

Figure 62. Clinial management of a shortened cervix

Cervical funneling

Another important parameter when assessing the cervix is funneling, which refers to the protrusion of the amniotic membranes into the cervical canal and progressive dilation originating at the internal os. While some consider funneling an additional risk factor for preterm delivery, recent studies indicate that cervical length (functional length - which is the part of the cervix that is still intact), and whether the cervix is closed, are more significant predictors.

Figure 63. Description of funneling

Figure 65. Illustration of the “T” shape, meaning the absece of funneiling

Figure 66. Image demonstrating the “T” shape, and no funneling seen

In the figure below (Figure 67) we see some fluid in front of the fetal head (indicated by the red arrow), but this is this is not funneling, since the internal os is contiguous with the lower uterine segment and is not open. The blue line indicates that the internal os is not funneling.

Figure 67. Image of a term cervix showing some fluid in front of the fetal head, but no funneling of the cervix

Below (Figure 68) are some similar images showing no funneling of the internal os.

Figure 68. Two images of cevix without funneling.

With increased pressure, the internal os begins to open, and the cervix has a “Y” shape.

Figure 69. Illustration of first stage of funneling, called the “Y” shape

Sometimes there appears to be a Y-shaped opening of the internal os with increased pressure applied to the cervix.

Figure 70, Image of a cervix with “Y” shaped funneling (REPLACE WITH TA IMAGE)

FIgure 71. Image showing measurement of the functional length of the cervix with funneling

The next stage is called the V-shape, where the internal os is open most of the way to the external os, leaving very little intact cervix

Figure 72. Illustration of funneling known as the “V” shape

The next step in the progression is the “u” shape. This is where the external os is stretched as well. This occurs whent the uterus is beginning to retract the lower uterine segment as a consequence of uterine contraction.

Figure 73. Illustration of “U” shaped funneling

Below (Figure 74) is an example of U-shaped funneling, with only a tiny part of the external os still intact (indicated by the small dotted line just to the left of the abbreviation “CX OS”.

Figure 74. Image of an example of “U” shaped funneling. (REPLACE WITH TA IMAGE)

Distinguishing true cervical funneling from its appearance during a uterine contraction can be challenging. In Figure 75 the cervix appears closed and long, so what appears to be a funnel shape may very well be from a uterine contraction compressing the myometrium just to the left of the area on the image. Continuing observation over time may help clarify if funneling is present.

Figure 75. Image of what might appear like funneling. (REPLACE WITH TA IMAGE)

Below (Figure 76) is a transabdominal view of a cervix. Although one might look at this at first and think it is a U-shaped funneling, look at the functional length of the cervix – it appears intact all the way up to the internal os. The dark bulge between the fetal head (left) and the cervix is a pocket of fluid that has collected in front of the fetal head. One thing to note is that the bladder is elongating the cervix, so the cervix may actually be shorter than it appears.

Figure 76. Image showing questionable funneling

The images below (Figure 77) display short cervices: 1.9 cm on the left and 1.0 cm on the right. The left image shows a near-term fetus, making a short cervix less significant, while the right image depicts a second trimester fetus, where a short cervix is more clinically relevant.

Figure 77. Images of short cervices (FIND OTHER TA IMAGES)

SUMMARY OF CERVIX SECTION

The rationale for measuring the cervix is to determine abnormal shortening to detect the potential of preterm labor in women who are at risk.

Cervical length can be initiated as soon as the cervix becomes differentiated from the uterus (not before 14 weeks’ gestation).

Screening of high-risk women (with prior history of preterm labor or birth) should occur between 16 and 24 weeks.

Screening after 24 weeks in asymptomatic women provides limited clinical value and does not appear to improve outcomes.

The cervix is ideally measured with relatively empty bladder if using a transabdominal or transperineal approach

Normal cervical lengths are:

< 22 weeks: 40 mm
22-32 weeks: 35 mm (10^th percentile is 25mm or 2.5 cm)

Cervical funneling can imply contraction activity and risk of preterm labor.

The practitioner must consult or refer for higher level management if a cervix is deemed to be shortening, especially in early second trimester.

It is noteworthy that short cervical length is not always associated with preterm labor or birth; the integrity of the functional length determines whether there will be early dilation.

CORD

This section describes fetal umbilical cord section characteristics. It reviews how to locate and identify the cord, either in 2-D mode, or with color Doppler. Detecting its presence can help with fluid pocket measurement of amniotic fluid (discussed in the next chapter). Certain characteristics can be noted that may have impication for fetal risk. Noting its location can help in detecting risks such as vasa previa.

By the completion of this Section the participant will be able to:

1. Describe 3 reasons why it is important to identify the umbilical cord

2. Describe clinical implication of a straight versus a twisted umbilical cord

3. Describe in general terms how doppler ultrasound is used to monitor blood flow to babies at risk for compromise

4. Interpret the findings of UA doppler in making a clinical management decision

Identifying the cord

Thre three major reasons for identifying and locating the cord during a POCUS exam are:

Avoiding amniotic fluid pockets that contain cord during performing AFI in a biophysical profile.
Observing for presence of cord in funneling
Observing cord or blood vessels at the cervix near term, between the presenting part and the cervical os

Figure 78. Importance of identifying the umbilical cord

An easy way to identify cord vessels is to look for the “Mickey Mouse” sign while transecting the cord transversely, showing two smaller arteries above a larger vein. Use color flow to confirm, as these areas will show red and blue for blood flow. The color of the 2 outer cord arteries will be one color, while the larger vessel (the umbilical vein) will be the opposite color. This is because the blood flowing through the arteries is the opposite direction of the blood flow in the vein. An easy way to remember the number of cord vessels is to think of the acronym “AVA”.

Figure 79. How to identify the umbiical cord

Figure 80 shows 2 images of cord. In the top image, the cord cross-section shows a 3-vessel cord (one larger vein in the center and the 2 smaller arteries on either side. The 2 arteries are pointed out with the red arrows. The bottom image displays twisted loops (green arrow), indicating cord loops. If uncertain whether these loops represent cord, activate color flow for confirmation, and the loops will display red and blue colors.

Figure 80. Images of cord using 2D or Bmode ultrasound

Figure 81. Cross section of a cord showing use of color Doppler

During point-of-care ultrasound, visualizing the number of blood vessels in the cord is usually unnecessary if the woman has had an anatomy ultrasound prior to your exam. Evaluating the umbiical cord would have been part of a standard fetal anatomy scan.

However, if you are the first person to scan her, try to establish 3 cord vessels. If only 2 are seen, or if you are unsure, refer for a more comprehensive ultrasound.

A 2-vessel cord (one artery, one vein) occurs in roughly 1% of pregnancies; if found mid-pregnancy, further screening for fetal abnormalities is recommended. Although a single umbilical artery does not put the fetus at risk, about 20% of cases may have related anomalies—mainly cardiovascular, GI, or renal. However, 80% demonstrate no concerns. There is also concern that with only 2 umbilical vessels there could be compromise to the fetus in other ways such as limiting growth. So follow up shoud be provided for these women.

Figure 82. Image and description of single umbilical artery

Cord cysts

Cystic structures may be seen on the cord and typically represent benign processes arising from Wharton’s jelly bulges that do not affect fetal circulation. However, in some cases they may represent a thrombus in one of the vessels. A diagnostic ultrasound should be ordered when a cyst is noted.

Twisted versus smooth cord

When scanning—even at point-of-care—observe the cord's twist, which is most visible with color flow. This usually signals fetal activity, correlating with good fetal health.

Figure 83. Images of twisted umbilical cord

A smooth cord, especially if noted in third trimester, is concerning, indicating that baby may not be very active. If this is seen, then consultation for further testing or ultrasound is appropriate.

Figure 84. Smooth umbiical cord (insert image)

Cord near cervical os

The image in Figureshows that there is cord near the cervix. If this is seen in second trimester, this may not be a problem, since the fetal head has not yet descended. If the pregnancy is near term, however, and the fetus is not in a vertex lie, this could represent a potential for a cord prolapse at time of labor or rupture of membranes. So consultation or follow up is recommended.

Figure 85. Image of cord near cervical os

Vasa previa

Vasa previa is not a common occurence, but when it is seen it most often is associated with a velamentous insertion and low-lying placenta. As you recall from earlier, the umbilical vessels insert into the membranes rather than directly into the placenta or cord.

It can also be associated with blood vessels traversing from the main part of the placenta to another placental lobe somewhere else in the uterus. This can cause the blood vessels traveling from the main placenta to the lobe to loop close to and overly the cervix. It is not cord since it does not travel to the fetus. As a result, these vessels may cross between the presenting part and the cervix. This condition is associated with an increased risk of vessel rupture during membrane rupture, which can lead to profound fetal blood loss. This is a dire condition and can lead to almost immediate fetal exanguination and intrauterine death. So it is important to detect this and either avoid labor or artificial rupture of membranes.

In cases of true vasa previa, rupture of a blood vessel can lead to significant fetal blood loss prior to the possibility of an emergency caesarean section.

Figure 86 (below) shows the presence of either a cord or blood vessels across the cervical internal os. More advanced ultrasound is indicated and consultation may be appropriate, depending on gestational age and the sonographer’s evaluation. If the pregnancy is near term, additional attention may be required.

Figure 86. Image of vasa previa identified with color flow covering the cervical os

Placental cord insertion

Part of the standard fetal anatomy scan includes evaluation of the umbilical cord’s insertion into the placenta. A central insertion is normal and ideal, while a marginal insertion may sometimes indicate reduced blood flow and possible fetal growth issues. However, as noted previously, marginal insertion may or may not be linked to adverse perinatal outcomes. When placentas are inspected post-delivery, it is common to see what is known as a “Battledore” configuration of the placenta. A Battlefore placenta is essentially a marginal or peripheral insertion of the cord. Those of you who deliver babies know what this looks like. As stated before, it would be good to know this prior to delivery to avoid putting undue tension on the cord when delivering the placenta. Because the cord is near the placental edge, it is more likely to shear off and result in having to do a manual extraction of the placenta.

Figure 87. Cord insertion into the placenta

Figure 88. Mid placental cord insertion (left image) versus marginal cord insertion (right image)

SUMMARY - CORD

The umbilical cord is the lifeline to the fetus, carrying nutrients and oxygen.

It is comprised of one umbilical vein, carrying maternal blood to the fetus, and two umbilical arteries, carrying blood from fetus to mother.

The cord structure is evaluated in the standard anatomic scan, and is NOT part of the routine POCUS exam. However, the location of the cord is very important to note when performing a POCUS exam to rule out:

-Low-lying cord

-Vasa previa

Although a POCUS exam does NOT include evaluation of fetal Doppler of the umbilical vessels, it is essential that the midwife understand the rationale for such testing and refer the woman for this testing if indicated.

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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 10. Maternal-Fetal Structures: Placenta, Cervix, Cord