Chapter 3. Fetal Biometry: Abdomen and Femur Measurements

Welcome to the chapter on fetal abdomen and femur biometry. This review will describe key abdominal landmarks for accurate measurement and highlight common mistakes to avoid.

Objectives:

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

1. Identify the landmarks used to establish the correct plane for measuring the fetal abdominal circumference

2. Place calipers correctly on the abdomen to create an ellipse for measuring the fetal abdominal circumference

3. Identify fetal kidneys

4. Describe ways to optimize the image of the fetal abdomen

BIOMETRY: ABDOMEN

Identifying landmarks

The key anatomic landmarks needed to confirm correct plane for scanning is shown in the diagram below (Figure 1.)

Figure 1. Simple diagram of important landmarks to identify when viewing the fetal abdomen.

Figure 2, Image 1. Comparison of diagram with image of fetal abdomen.

Abdominal circumference landmarks ^{1, 2, 3, 4, 5, 6}

This ultrasound image presents a transverse section of the abdomen. In this instance, the fetal spine is aligned toward the maternal spine, with the stomach positioned on the left side of the screen, indicating that the fetal head is directed away from the observer. The image also reveals two ribs adjacent to the vertebrae. Observing one set of ribs is considered acceptible; however, if multiple ribs are visible and create shadowing, it suggests that the view was obtained at an oblique angle to the abdomen rather than straight across.

Image 2. Abdominal circumference landmarks are annotated.

Figure 2 outlines the steps to locate the correct plane for measuring abdominal circumference. Ensure the fetal kidneys are not visible; seeing them means the transducer is too low. Although kidneys can be hard to detect, they appear in this image, indicated by the red dotted lines.

Figure 3, Image 2.

How to find the AC^{1, 7}

First, identify the baby’s head and obtain a sagittal view of the spine, matching the reference ultrasound image. Center the fetal stomach on the screen, then rotate the transducer so the notch points to the woman's right. This will provide a transverse abdominal view with the stomach visible. Adjust the transducer’s position or angle as needed to view other structures.

Next, check that the entire abdominal edge is visible. The fetal stomach is anatomically on the baby's left when the head faces you; if the baby is turned, the stomach may appear on the other side. This does not indicate abnormal placement, but rather the baby’s position. Real-time scanning and observing other structures will also help determine orientation.

Figure 3 below illustrates the above description of how to find the AC.

Figure 3., Images 2, 3. Illustrations showing how to track the fetal spine sagittally to loate the stomach, then rotating the probe to get the transverse view to measure.

Measurement of the AC

To measure abdominal circumference, select the AC button, place the caliper at the skin's edge near the vertebrae, and press the button again to create an ellipse. Adjust the cursors as needed for a closer fit. The ellipse should be placed on the outer line of the skin all the way around the abdomen. Once satisfied, hit "set" for calculation, then save or store the image and measurement.

NOTE: the abdomen should be almost perfectly round unless it is imaged obliquely, or if the practitioner is placing too much pressure on the transducer, squeezing it into an oval shape.

This model illustrates the location in the fetal torso where the ultrasound "slice" transection occurs, specifically above the umbilicus and below the heart.

Figure 4. Photo of a 3D model of a fetal abdomen with the line indicting the location of transection with ultrasound.

The following figures demonstrate how the ultrasound beam should transect the fetal abdomen.

Figure 5. Illustration of how the ultrasound beam should transect the fetal abdomen below the heart.

The transverse slice should also be done above the umbilical cord insertion (Figure 6.)

Figure 6. View of the angle of transecting the fetal abdomen above the umbilicus insertion.

The plane also needs to be above the kidneys, as described previously. This is illustrated in Figure 7.

Figure 7. Transecting the fetal abdomen above the kidneys.

Images of AC at progressive gestations

The early images of the AC at 14 weeks (below) shows that landmarks are hard to distinguish. The vertebra appears faintly in the left image and more clearly in the right; a small stomach is visible in the right, but the portal vein is not seen.

Image 4. AC at 14 weeks’ gestation.

At 17 weeks, more detail of the abdominal organs is evident.

Image 5, 6. AC at 17 weeks’ gestation.

Between 18 and 20 weeks, the spine and stomach are generally visible, while the portal vein may not be clearly observed.

Images 7, 8, 9. AC at 18 to 20 weeks’ gestation

Between 23 and 25 weeks, it is much easier to see all the landmarks, including the portal vein.

Images 10, 11, 12. AC at 23 to 25 weeks’ gestation.

From 27 to 30 weeks, all fetal landmarks are clearly visible. As the fetus grows, the placenta may obstruct the abdominal circumference, requiring estimation with the ellipse tool where the edge extends, even if it overlaps the placenta. Like measuring head circumference, you can adjust the depth to fit the image within the sector.

Images 13, 14, 15. AC at 27 to 30 weeks’ gestation.

In the image on the right (below, image 16) , the imaging depth was adjusted to accommodate the entire abdomen within the sector. Additionally, there is minimal fluid present in this image, resulting in decreased clarity and increased blurring. Although the 32-week abdomen depicted on the right appears smaller than the 30-week abdomen on the left, this difference is attributable to the alteration in ultrasound depth settings, done to ensure complete visualization of the abdomen.

Images 16, 17. AC at 30 to 32 weeks’ gestation.

As the fetus grows, getting a perfectly circular view of the abdomen becomes difficult. The ellipse may need to fit as closely as possible, even if imperfectly. The image shows the vertebra is not a true cross-section because the curved fetal back makes it hard to align the stomach and portal vein with the vertebra in one plane.

Image 18, 19. AC at 34 and 38 weeks’ gestation.

Anatomic variation depends on how the fetus is lying

This image shows that the fetal stomach's position can look different on ultrasound depending on the baby's orientation, even though it is always on the baby’s left side in reality.

The image shows the baby's stomach on its right side, indicating the head is turned away from us. With the spine down, the baby is facing up. Using a fetal torso or doll can help visualize organ positions when learning this.

Figure 8, Images 20, 21. 3-D models and corresponding images to illustrate how the view of the stomach differs depending on fetal position.

Pitfalls of measuring AC

The next series of images highlight common issues with abdominal circumference measurement. In image 22 (below) the spine is positioned anteriorly, casting a shadow that blocks visualization of structures located behind it.

Image 22. Abdomen view with kidneys seen. Also there is a large acoustic shadow from the anterior vertebrae, obscuring the anatomy below the vertebrae.

Also seen in this image is a cross-section of both kidneys, demonstrating that the slice in the abdomen is too low on the fetal torso (kidneys are lower than the plane used to view the AC). To correct this, slide the transducer more toward the baby’s head.

These two images show excessive pressure on the abdomen during scanning, distorting its shape and affecting HC measurement and growth calculations. Apply enough pressure to obtain a clear image but avoid compressing the abdomen.

Images 23, 24. AC measurements: image on the left does not display the correct landmarks. Image 24 (right) shows the baby’s abdomen somewhat compressed between the uterine wall and the posterior placenta, possibly rendering an inaccurat measurement.

Seeing multiple ribs and shadows indicates a tangential, not transverse, view of the abdomen.

Images 25, 26. Views of fetal abdomen showing multiple ribs, which indicate tangential angle of incidence rather than a 90% transection.

Difficult visualization

A potential cause of suboptimal imaging is insufficient amniotic fluid. In this situation, it is important to examine the fluid throughout the uterus to assess for generalized low fluid volume, or whether low fluid is seen only in the area of interest. If adequate fluid exists elsewhere in the uterus, repositioning the patient onto her side may redistribute the fluid toward the fetal abdominal region. However, if oligohydramnios is observed, this warrants concern and consultation with an advanced healthcare provider is advised.

Image 27. Fetal abdomen not clearly seen due to lack of fluid to enhance the image.

Inaccurate images/measurements

These images show poor technique. The left image has excessive spinal shadow and lacks key landmarks except for a small stomach. The right image includes the heart, indicating an incorrect angle that is too high in the abdomen, and crossing into the chest.

Images 28, 29. Images of fetal abdomen demonstrating poor technique. Image 28 (left) has large shadow obscuring abdominal contents, and image 29 (right ) is oblong instead of round. Also, the fetal heart is seen, indicating the angle of insonation goes into the fetal chest.

Not seeing the fetal stomach

The fetal stomach isn't clearly visible in image 30 (below), likely because it's empty during the scan. Waiting a few minutes may allow it to fill so it can be seen.

Image 30. No fetal stomach identified in abdomen.

This is an image of an abdomen that is certainly not round. The image has obviously been taken at a tangent, given that multiple ribs and rib shadows are seen at the edge. This makes the measurement inaccurate.

Image 31. Incorrect angle of insonation across the abdomenEX

Artifact example

The image below (Image 32) shows what appears to be a very large fetal stomach. This is unlikely. The observed dark area is most likely a shadow cast by the vertebrae, which may obscure some or all of the stomach's typical appearance. To confirm this finding, adjust the imaging angle or reposition the woman to facilitate fetal movement. This causes the spine to move from the position that casts the shadow downward. Then verify that the stomach size is within normal limits.

Image 32. Fetal abdomen with enlarged stomach, but caused by shadow cast by the vertebrae above the area. Also, an “edge” shadow is coming from the side of the fetal abdomen.

Image adjustment

To improve this image, include the portal vein in the anterior of the fetal abdomen. The kidneys are visible lateral to the vertebrae, while ribs seen near the spine suggest an oblique angle. The spine's shadow could be reduced by adjusting the transducer angle. Additionally, the elliptical shape of the abdomen indicates excessive compression; it should appear round.

Image 33. Image of AC showing 2 problems: one is the ovoid shape of the abdomen (caused by transducer pressure) and the other is a large shadow from the vertebrae, obscuring the portal vein.

The image is too dark to view the baby's stomach or left abdominal area. Increasing the gain will make it brighter and easier to recognize landmarks.

Image 34. AC landmarks obscured by too little gain (image is too dark)

Abnormal abdominal finding: echogenic bowel

This image shows a “soft” marker called echogenic bowel, where the baby’s bowel appears unusually bright on ultrasound; sometimes as bright as bone. While mild increased echogenicity is common, pronounced brightness can indicate risks such as aneuploidy, cystic fibrosis, bowel obstruction, or infection. If seen, a detailed sonogram is recommended. The significance depends on how bright the bowel appears. Formal diagnostic scanning applies estimation of brightness gradations, with brighter images correlating with increased risk.

Image 35, 36. Images of increased bowel density called echogenic bowel.

SUMMARY ABDOMEN

Landmarks used to establish the correct plane for measuring the fetal abdominal circumference are: fetal stomach, umbilical vein/portal vein, and transverse view of vertebrae.

Correct caliper placement for abdominal circumference is on the outside of the fetal skin, creating a symmetrical or circular ellipse around the abdomen.

To assure an accurate measuring plane, the following structures should NOT be seen in the view:

Fetal kidneys

Fetal umbilical cord insertion

Fetal heart

Multiple rib shadows

Artifacts may mislead abdominal landmark identification. To prevent this:

Change the angle of insonation

Increase or decrease gain

Increase or decrease depth or focal zone

Turn the woman on her side to encourage fetal repositioning

BIOMETRY: FEMUR

Objectives

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

1. Locate the correct angle of the fetal femur (in its longest dimension)

2. Discriminate between the diaphysis (shaft) and epiphysis of a fetal femur

3. Place calipers correctly on the femur for accurate measurement.

How to find the femur

First, identify the baby’s head and scan sagittally along the spine toward the fetal legs. When you reach the pelvis, look for a bright dot with a shadow, indicating a transverse view of the femoral shaft. Center the image, then rotate the probe until the femur appears fully extended and both ends are clearly defined.^{1, 7}

Figure 9. Images 37, 38. Illustration of tracking the fetal spine to the pelvis, then rotating the probe to locate the long view of the femur.

Femur length: general principles

The best time to measure the femur is between 15 and 32 weeks of gestation, as normal length varies more after 32 weeks. Use a horizontal view (perpendicular to the shaft of the femur) for optimal sonographic imaging. If you see irregular or hypoechoic extensions on the shaft, they are likely part of the epiphysis and should not be included in your measurement.

Figure 10. General principles of measuring femur length.

Figure 11. Additional tips for measuring femur length

Choose the femur closest to your transducer for better accuracy. Adjust the transducer so the diaphysis appears straight. Select FL, place the cursor at one end of the femur, hit set, then drag the second cursor along the shaft to the opposite edge.^1,7

FL image and illustration

This is an image of a femur length, and an illustration of it on the right.

Figure 12, Image 39. Illustration and image of measuring a fetal femur length.

For precise measurements, only the ossified shaft of the femur should be considered.

In the image below at 35 weeks' gestation, the femur displays round hypoechoic epiphyses on either side of the diaphysis, which are less visible earlier in pregnancy.

Image 40. Image showing correct caliper placement on the shaft of the bone, and not extending to the area just above the epiphysis.

In the 19-week fetal image below, the hypoechoic area at the knee is barely visible and easily overlooked.

Image 41. Image showing FL not including the epiphysis.

For greater accuracy when viewing the fetus, position the femur horizontally at a 90-degree angle of insonation.

Image 42, 43, 44. Images of FL at 15 to 16 weeks’ gestation.

The image on the right (Image 42) is most technically accurate, showing the top of the femur in a horizontal plane. The left image measures the lower FL, which can generally be less precise than measuring the top femur; while the middle image angles the femur downward, making its measurement unreliable because of the inability to approach the femur at 90 degrees.

Femur lengths at 18-19 weeks show clearer bones with increased echogenicity.

Images 45, 46. FL at 18-19 weeks’ gestation.

These are images from 22-23 wks

Images 47, 48, 49. Femur lengths at 22-23 weeks’ gestation.

Presented below are two supplementary images taken at 24–25 weeks. Please note that in the image on the right, the placement of the dotted caliper line is slightly inaccurate, as it is positioned adjacent to the femoral shaft rather than directly on it.

Images 50, 51. FL at 24 to 25 weeks’ gestation.

At 27-28 weeks, the femur length is beginning to fill up the sector

Images 52, 53. FL at 27-28 weeks’ gestation.

Images 54, 55. FL at 30-31 weeks’ gestation.

At 36–40 weeks, the femur can be hard to capture in one view because of its size and position, particularly if it is close to the mother's abdominal surface near the probe.

Images 56, 57 FL at 36-40 weeks’ gestation.

Low amniotic fluid

This shows a femur length with low fluid. Its density makes it easy to visualize, reflecting a bright image despite minimal fluid.

Image 58. Low fluid making clear view of the femur difficult.

Pitfall: measuring the wrong bone

In Image 59, the baby's position is not clearly identified, so it is uncertain whether the visible long bone is a humerus, with the fetal head to the left, or a femur, with the fetal head to the right. It is important to consider that the baby’s legs may be positioned near the face, which can lead to confusion between a humerus and a femur. Additionally, the tibia may sometimes appear instead of the femur; therefore, confirming the presence of a second parallel bone can help ensure accurate identification of a tibia and adjacent fibula.

Image 59. Measuring the incorrect bone

Pitfall measuring linear projections at the end of the femur

Image 60. Measurement of the bone shaft and NOT including the projection at the end.

This image indicates the area of the femur that should not be measured; measuring this projection at one end of the femur is not recommended.

Suboptimal measurements

The calipers in these images are not aligned with the femoral shaft, illustrating improper measurement technique.

Images 61, 62. Suboptimal measurements of FL: placing a straight line on the curved portion of the femur; remedy is to image from a different direction to allow view of a straight line.

SUMMARY: FEMUR LENGTH

To locate the fetal femur, align the transducer in a sagittal view along the fetal spine toward the pelvis. When at the pelvis, rotate the transducer to visualize the femur(s). Measure the femur nearest the transducer, ensuring it is horizontal on the screen for optimal accuracy.

Measure femur length along the longest part of the fetal shaft, aligning calipers with the shaft and excluding hypoechoic epiphyses. Mild femur bowing usually results from angle of insonation; marked bowing requires further assessment.

Women's Point-of-Care Ultrasound: Fetal Biometry and Growth

Chapter 3. Fetal Biometry: Abdomen and Femur Measurements