Chapter 4. Maternal/fetal Structures: Placenta and Cord

Welcome to the chapter on ultrasound of the placenta and umbilical cord. This chapter will review describing placental location and characteristics. Parameters that define a low-lying placenta as well as placenta previa are discussed. In addition, the classification of placenta accreta conditions known as PAS or Placental Accreta Spectrum is presented. Ev

Evaluation of the umbilical cord is included in the standard 20-week fetal anatomy scan, but it is prudent even during a POCUS scan to look at the location and characteristics of the cord.

OBJECTIVES

By the completion of this Section 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 technique to identify
Differentiate between uterine fibroid and uterine contraction
Describe the types of placentation in twin gestation
Describe the categories of placenta accreta spectrum (PAS)

Some interesting facts about the placenta are:^1,2

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

Figure 1. List of facts about the placenta.

Reasons for evaluating the placenta

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. List of 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 placental image

Figure 3. List of techniques to obtain an optimal placental image.

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 4., Image 1. Imaging the placenta with color.

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.

ISUOG guidelines for evaluating the placenta³

Figure 5., Images 2, 3. ISUOG guides for imaging the placenta in the first trimester.

ISUOG 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 between 11-14 weeks

Placental size:⁴

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

Triploid

Conditions associated with small placenta

Intrauterine growth restriction

Abnormal placentation

Preclampsia

Severe insulin-dependent diabetes

Chronic infection*

Chromosomal abnormalities

Placental thickness ranges from around 1 cm at 10 weeks gestation, to 2 cm at 20 weeks, and 3.25 by term.¹

Heeralall et al⁵ studied placental pathologies linked to COVID, identifying numerous distinct variations and abnormalities. Many of these effects are related to inflammatory processes affecting placental/fetal circulation.

Placenta Previa

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.^6,7

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.

Localization of placenta:

Knowing the placenta’s location helps identify risks like low placement and abnormal bleeding, ensures safe procedures such as insertion of intrauterine monitoring devices during labor, and aids management in case a C-section is needed.

Figure 6. Evaluating placental position.

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). 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.

Essential components of placental evaluation

Figure 7. Essential components of placental evaluation.

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

-Observing Placental Location⁴

-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 8, Image 4. Anterior placental location.

Figure 8 (above) demonstrates an anterior placental location (toward the anterior uterine wall)

Figure 9, Image 5. Posterior placental location.

The image above in FIgure 9 demonstrates a posterior placenta (toward the maternal spine)

Image 6. Transverse view of the anterior placenta, demonstrating that most of the placenta was toward the maternal right.

Image 6 is transverse view of the mother’s pelvis, showing that the placenta is anterior, mostly positioned on the mother's right side.

Image 7. Image of anterior placenta and relationship to maternal spine.

Image 7 is another illustration of an anterior placenta as it lies closest to the maternal skin line and the transducer.

Image 7. Image of anterior placenta and relationship to maternal spine, annotated.

The red line (above) indicates the outline of the placenta.

Image 8. Image of posterior placenta curling up toward the fundus.

In this sagittal view in Image 8, 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).

Image 9. Transverse view of the placenta, demonstrating the placenta was curled toward the maternal left.

Image 9 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 left/anterior.

Image 10. Sagittal view of a fundal placenta.

Image 10 is a sagittal view of the mother, demonstrating that the placenta is up 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

Image 11. Sagittal view of fetus in breech presentation with anterior placenta.

The left side of the screen in the image above (Image 11) shows the green dot, or indicator, corresponding to either the mother's head or her right side. Because this is a sagittal view of the mother's abdomen, the baby's head is positioned toward her head, and the placenta is located anteriorly.

Placenta grading⁴

Previously, sonographers assessed placental maturity by examining its fetal surface and echogenicity, using changes in texture as indicators aligned with gestational age. This 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. Siargkas et al⁷ maintains that placental grading remains relevant to predicting pregnancy outcomes (Table 1.).

Table 1., Images 12, 13, 14, 15. Placenta grading criteria and images.

Placental hematomas

Figure 10 below describes various types and locations of placental hematomas. Subchorionic hematoma is frequently observed during early pregnancy, and in some cases, persists into later gestation.

Figure 10, images 16,17,18. Illustration and image examples of placental hematoma.

Placental lakes

Placental lakes are primarily 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 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.

The number of lakes does not appear to be problematic, but the size may represent a problem if it exceeds 5 cm.

Figure 11. Description of placental lakes.

Image 19 (below) shows placental lakes on the fetal size of a posterior placenta.

Image 19. Image demonstrating venous lakes.

Venous lakes are also sometimes called lacunae. As seen in these images, they can be located on the fetal side of the placenta, or within the placenta.

Low-lying placenta

Discussion will now focus on low-lying placenta and placenta previa. 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.

Figure 12. Definition of low-lying placenta.

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 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

Scanning a low lying placenta

Figure 13. Imaging a low-lying placenta.

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 to the anterior lip of the cervix may distort the lower uterine segment and affect the accuracy of the measurement.

Image 20. Image of a posterior low-lying placenta and relationship to internal cervical os.

Image 20. Image of a posterior low-lying placenta and relationship to internal cervical os, annotated.

Image 20 (above) displays a red arrow marking the lowest edge of the posterior placenta, and a blue arrow indicating the indentation at the internal cervical os.

To determine if this is a low-lying placenta, the distance between the specified points is measured at 2.5 cm. 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.

Image 21. Anterior low-lying placenta.

The image above (image 21) shows an anterior low-lying placenta. The internal os distance (IOD) was not recorded, likely because the anterior edge is positioned at the margin of the internal os.

Image 21. Anterior low-lying placenta, annotated.

The red lines in the above image demonstrate the edge of the placenta encroaching on the internal cervical os.

The case below (image 22) demonstrates a low-lying posterior placenta with a measured distance of 2.2 cm, which does not meet the technical criteria. However, there is apparent cervical funneling, indicating that a transvaginal measurement would provide greater accuracy, as this image was obtained via a transabdominal approach.

Image 22. Image of posterior placenta previa, overlapping the internal os.

Image 22. Image of posterior placenta previa, overlapping the internal os, annotated.

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

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. There is thought that placentas do not actually move, but that the lower edge of placentas degenerate due to less blood flow in the lower uterine segment, essentially appearing to move.

Figure 14. Studies citing incidence of resolution of low-lying placentas

A 2020 study by Jansenet al⁹ found 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 15. Additional information about resolutin of low-lying placenta

The illustration below demonstrates what would be a low-lying placenta

Figure 16. Illustration of low-lying placenta

Image 23 shows a low-lying placenta

Image 23. Image of low-lying placenta

Image 23. Image of low-lying placenta, annotated.

The red line in Image 23 indicates where the lowest edge of the placenta comes in relationship to the internal cervical os.

Image 24. Image of anterior low-lying placenta using transabdominal scan.

Image 24 is another example of 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.

Image 24. Image of anterior low-lying placenta using transabdominal scan, annotated.

The red dotted line indicates the edge of the placenta (red arrow) and the barely visible internal os

Terminology

In the past, terms were designated to describe various degrees of placenta previa (see Figure 17). These terms may still be encountered in some contexts; however, they are no longer regarded as appropriate descriptors Instead, a more accurate and acceptable method of documentation is the measured distance from the leading placental edge to the internal cervical os – the IOD.

Figure 17., Images 25,26,27. Examples of prior terminology describing categories of low-lying placentas and placenta previas.

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 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

Figure 18. Description of placenta previa

Some other things to note while observing the placenta are focal uterine contractions. A very full bladder can also distort the appearance of the placental relationship to the uterus and cervix.

Figure 19. Factors that mimic placenta previa.

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.

Image 25. Image of anterior placenta laying adjacent to a uterine contraction.

The red line in image 25 (below) highlights the location of the focal uterine contraction. Uterine contractions can occur frequently, including during the first or second trimester. These contractions are not indicative of labor. It is appropriate to inform the mother or family members observing the scan that these findings do not suggest any problems for the baby, nor does it increase risk of preterm labor.

Image 25. Image of anterior placenta laying adjacent to a uterine contraction, annotated.

Management of 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.

Figure 20. Management of placenta previa.

Placenta Cord Insertion

ISUOG now recommends assessing cord insertion into the placenta, typically during the second-trimester anatomy scan or via POCUS if no prior scan exists. Identifying whether the cord is near the placental edge is important, as this may reduce fetal blood flow and increase the risk of growth restriction.

It also can present a potential risk for problems in third stage of labor. The placenta may have to invert itself during any cord traction, and this could contribute to cord avulsion. When this happens then manual removal of the placenta may become necessary.

A recent study by Lutz et al⁹ found no link between marginal cord insertion and adverse perinatal outcomes, but there are cases where a marginal insertion can affect fetal circulation.

Figure 21, images 26, 27, 28. Images demonstrating placental cord insertion variations.

Placental cysts

Placental cysts are hypoechoic, cystic structures distinct from placental vasculature, typically found on the fetal aspect of the placenta. These lesions may present as round or oval and can be subchorionic or intraplacental in location. Particular attention should be paid to cysts situated near the umbilical cord.

Figure 22, Image 29. Placental cyst description and image.

Placenta chorioangioma

Placental chorioangiomas are vascular tumors found in the placenta and typically manifest as irregular masses within the placental tissue. They are typically located near the fetal surface of the placenta. These tumors occur infrequently (about 1 in 100 deliveries) and are typically benign. The pregnancy is not affected unless they are very large.⁴

Image 30. Placental chorioangioma.

Vasa Previa

Vasa previa most commonly occurs when the umbilical vessels insert into the membranes rather than directly into the placenta. As a result, the 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.

Figure 23, Image 31. Image and description of vasa previa.

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.

In an asymptomatic woman, it could possibly be noted 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.

Figure 24. Description of placental abruption.

Risk factors for placental abruption:

Hypertension

Smoking

Drug/cocaine use

Trauma

Short umbilical cord

Submucosal fibroids

Uterine anomalies

Placenta previa

Premature rupture of membranes

Figure 25. Findings associated with placental abruption.

Placental abruption may occur with retroplacental myoma, myometrial contractions, or placental calcifications. If the fetus is healthy and there are no maternal symptoms, follow-up ultrasounds are recommended.

Figure 26. Placental abruption information.

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.

Image 31 below demonstrates placental separation and relative hypoechogenicity between the placenta and uterine wall. The next slide will highlight this with arrows.

Image 31. Image of placental abruption.

Image 31. Image of placental abruption, annotated.

The blue arrow in image 31 indicates the placenta. The hypoechoic 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.

Placenta 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.

Figure 27., Images 32, 33. Variations of placental configurations with image illustrations.

Circumvallate placenta

This image and diagram in Figure 28 (below) demonstrate the “shelf” often seen with a circumvallate placenta.

Figure 28, Image 34. Illustration and image of circumvallate placenta.

Succenturiate placental lobe

The diagram 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 29, Image 35. Illustration of succenturiate placental lobe, demonstrated in the image.

Uterine synechia

Uterine synechia, occasionally found near placental tissue, are uncommon and usually result from scar tissue after prior uterine surgery.

Image 36. Image of uterine synechia.

Placentation of twins

As described in the section on diagnosing twins, chorionicity can be determined chorionicity if there are two placentas with 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.

Figure 30. Illustration of twin placentation.

This reiterates the importance of determining chorionicity and amnionicity as early as possible in the first trimester

Image 37. Image of a dichorionic, diamniotic twin placenta, showing amnion.

In this pregnancy image (above) 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)¹⁰, with a current incidence of 1 in 272 births.

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.

This occurs because decidualization happens in 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.

Diagnosing a placenta accreta, increta or percreta is not typically within the scope of POCUS. However, it is reviewed in this chapter so learners can be aware of the potential of this dangerous condition in women who have had prior C/Section or uterine surgery. It is also important to understand this condition when reviewing standard ultrasound reports from outside radiology services.

Figure 31. Illustration of placenta accreta.

PAS categories diagram⁴(above in Figure 31)

This diagram 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.

Placenta increta occurs when placental tissue invades deep into the myometrium, preventing delivery of the placenta in the third stage and leading to subinvolution and postpartum hemorrhage. Surgical removal is necessary. Increta extends through the myometrium but does not penetrate through the outer uterine serosa.

Figure 32. Placenta increta.

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.

Figure 33. Placenta percreta.

PAS Management

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 specialist; often an MRI is an additional step in making the diagnosis.

Figure 34. Diagnosis and management of placenta.

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

SUMMARY

In summary:

Evaluation of the placenta is important to detect certain fetal abnormalities and predict the potential for maternal and fetal complications

Components of the placental exam include

Location

Size and appearance

Relationship to the cervical os

Presence of abnormal structures (cysts or hematomas)

Insertion of the cord

Presence of additional lobes

Placenta Previa is a common condition that has a high incidence of resolution. When it does not resolve, however, it needs to be evaluated near term to determine safest mode of delivery

PAS is a significant contributor to maternal morbidity and mortality and is categorized by degree of placental infiltration

This condition requires referral and management by a team of high-risk obstetric providers

SECTION 2: CORD

Welcome to the fetal umbilical cord section of this chapter. This section describes how to identify and locate the cord, review its variations, and discuss the use of color Doppler to detect potential placental perfusion issues.

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

Three reasons for identifying and locating the cord are:

1. Avoiding amniotic fluid pockets that contain cord during performing AFI in a biophysical profile.

2. Observing for presence of cord in funneling

3. Observing cord or blood vessels at the cervix near term, between the presenting part and the cervical os

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.

Figure 35. Identifying the umbilical cord.

Image 38. Image of 3-vessel cord in cross-section and long view.

Image 38 (above) shows two examples of fluid pocket measurements. In the top image, the cord cross-section shows the two arteries (red arrows) on either side of the vein. The bottom image displays twisted loops (green arrow), indicating cord loops. If uncertain whether it's cord, activate color flow for confirmation.

Image 39. Cross section of umbilical cord using color Doppler.

Image 39 uses color flow to highlight a cross-section of the cord.

Single umbilical artery

During point-of-care ultrasound, visualizing the number of blood vessels in the cord is not usually necessary, since it is part of a standard fetal anatomy scan and has likely already been noted. 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. If a 2-vessel cord is seen, a detailed ultrasound should be performed to check for other fetal abnormalities.

Figure 36, Image 40. Two-vessel cord identification and implication

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.

Images 41, 42. Images of umbilical cord twisting.

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 36, Image 43. Implication and example of smooth umbilical cord.

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

Nuchal cord

The image below presents a cross-sectional view of the fetus at the fetal cervical spine level. Occasionally, the umbilical cord may be observed looped around various fetal body parts, including the neck. It is well established that a nuchal cord is relatively common and does not necessarily predict complications during delivery. There is ongoing debate regarding whether patients should be informed of this finding in the absence of additional indications suggesting potential fetal distress.

Image 44. Transverse view of fetal neck, showing evidence of nucal cord with color Doppler.

Umbilical cord near cervical os

The image below (image 45) shows 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.

Image 45. Image of umbilical cord near os.

Vasa previa

Image 46 (below) indicates the presence of either a cord or blood vessels across the cervical internal os. Further follow-up or consultation may be appropriate, depending on gestational age. If the pregnancy is near term, additional attention may be required. 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.

Image 46. Image of vasa previa.

Cord insertion into placenta

As described in the first section of this course, an important part of the standard fetal anatomy scan includes evaluation of the umbilical cord’s insertion into the placenta. A central insertion is normal, 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.¹⁰ When placentas are inspected post-delivery, it is common to see what is known as a “Battledore” configuration of the placenta. This is essentially a marginal or peripheral insertion of the cord.

Images 47, 48. Images of placental cord insertion.

Images 49, 50. Placenta cord insertion examples: left image is central insertion, right image is marginal insertion toward lower placental edge.

Doppler evaluation of cord

As described in the Foundations Course in this series, Doppler ultrasound relies on the Doppler Effect principle: when a moving object reflects ultrasound waves, the echo frequency rises if it approaches the probe and falls if it moves away. The frequency change correlates with the object's speed. Doppler ultrasound uses these shifts to assess both the velocity and direction of movement, primarily measuring blood flow rates.

Doppler flow in the umbilical artery shows how much effort the baby’s heart needs to push blood into the placenta for oxygen. It reflects placental vascular resistance. Increased resistance, often from early placental deterioration or insufficiency, can cause intrauterine growth retardation and fetal compromise.

The numerical values presented in Figure 37 clarify how to interpret an umbilical artery evaluation report generated from an advanced ultrasound examination. The S/D ratio serves as a metric to determine the relative amplitude between systolic and diastolic measurements.

Figure 37. Description of umbilical artery Doppler.

What is the significance of these indices?

These measurements are generally considered not an appropriate evaluation in cases of normal pregnancy, so there is no need to order this test in that context. When used in the case of a high risk pregnancy, the key values to know are:

In the second trimester, normal ranges are from 1.5 to 2.0.

At term, typical values fall between 1.0 and 1.5.

Figure 38. Implications of umbilical artery Doppler

This diagram in Figure 38 (above) illustrates Doppler measurement of cord blood flow.

· S (systolic) refers to the maximum velocity of blood flow resulting from fetal heart contraction toward the placenta.

· D (diastolic) describes the ongoing forward flow toward the placenta during the relaxation phase of the fetal heart rate, indicating low resistance within the placenta.

If there is increased placental impedance, forward blood flow may cease or reverse direction, suggesting that the placenta resists blood movement toward it, and increases backflow toward the fetus.

Umbilical artery impedance indices rise when end-diastolic flow is decreased because of reduced placental perfusion and utero-placental insufficiency, such as in cases of intrauterine growth restriction (IUGR).

Figure 39, Image 51. Examples and description of interpreting Doppler

Uterine artery doppler

The uterine artery Doppler, described above, is often ordered in high-risk pregnancies. Abnormal UA Doppler results may predict complications like pre-eclampsia and fetal hypoxemia. While not part of a POCUS study, these findings are commonly included in radiology reports and you may need to interpret or apply the findings.

Figure 39. Description of UTERINE ARTERY Doppler evaluation.

SUMMARY

--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 the fetus to the mother

--the cord structure is evaluated in the anatomic scan and is not part of the routine POCUS exam; however the location of the cord is very important to asses 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 healthcare provider understand the rationale for such testing and be able to refer the woman for this testing if indicated

Women's Point-of-Care Ultrasound: Second and Third Trimester

Second and Third Trimester Overview