Objectives

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

Identify landmarks of the fetal skull and brain that correlate with the correct plane for imaging a biparietal diameter and head circumference

1. Understand correct caliper placement on the image to measure BPD and HC

2. Recognize incorrect planes or caliper placement on images

3. Define cephalic index and its meaning

The fetal head size, measured by the biparietal diameter (BPD) and circumference (HC) is the first parameter discussed in this course, and is often the first to be measured when performing a biometric exam. The more precise the measurements are, the more accurate the estimated gestational age and predicted date of delivery will be.

NOTE: POCUS enables faster assessments than traditional diagnostic exams, primarily serving as a triage tool for clinical decision-making, prioritizing speed over precision. Providers should still aim for maximum accuracy by following recommended ultrasound principles. These principles set measurement standards for all ultrasound practitioners.

Some tips for obtaining fetal head measurements are:

--BPD, HC, AC, and FL should be measured on ultrasound scan from 14 weeks onward1

--Enlarge the image to the widest that will be completely seen in the ultrasound screen sector

--Adjust the gain so that the internal brain structures used to identify the correct imaging plane are clearly seen

--If the head has a shape other than the normal ovoid or “egg-shaped” configuration, the head circumference is typically more accurate than the biparietal diameter.

--For those machines using an “AI” automated generation of the BPD or HC, it is still necessary to visualize and adjust the caliper or ellipse placement for accuracy.

--Estimated due date (EDC or EDD) should never be assigned to the fetus with a second or third trimester ultrasound if there is an ultrasound measurement obtained in the first trimester (which is more accurate).2

Ultrasound measures the fetal head by biparietal diameter (distance between parietal bone edges) and head circumference, both in the same plane.

Fetal brain

Ultrasound measures the fetal head by biparietal diameter (distance between parietal bone edges) and head circumference, both in the same plane.

Figure 1. Diagram of how to angle toward the fetal head for correct imaging.

NOTE: POCUS enables faster assessments than traditional diagnostic exams, primarily serving as a triage tool for clinical decision-making, prioritizing speed over precision. Providers should still aim for maximum accuracy by following recommended ultrasound principles. These principles set measurement standards for all ultrasound practitioners.

Fetal brain landmarks are diagrammed below in Figure 2.

Figure 2. Illustration of the fetal brain to be identified during a fetal head ultrasound.

When examining a cross section of the fetal brain, the following structures should be aligned:

Falx Cerebri (known as the falx)

Thalamus (there are two, one on either side of the falx, known as thalami)

Cavum Septum pellucidum (CSP) which lies in the frontal region along either side of the falx and has the appearance of a small “box” bordered with echogenic lines, anterior to the thalami.

Figure 3. Overlay of the fetal skull, correlating the skull structures to the brain structures.

The image overlay in Figure 3. displays the skull and suture bones over brain structures as seen on ultrasound, aiding visualization of how these structures relate to the skull.

Fetal head anatomy

Figure 4. Simplified illustration of the basic structures necessary to include in a fetal head evaluation.

This diagram shows how the structures are aligned to achieve the correct plane for measuring fetal head BPD and HC. Structures slightly above the BPD plane, such as the choroids, may appear in early measurements; these are lighter, cloud-shaped areas seen lateral and posterior to thalami.

BIPARIETAL DIAMETER (BPD)

The biparietal diameter (BPD) is measured as follows:

To measure BPD, locate the widest diameter between the outer edges of the biparietal bones. The fetal head is egg-shaped, not perfectly round or oval, so identify the widest point clearly. Ensure the head appears symmetrical and the falx is centered from front to back.

Also make sure all or most of your landmarks are visible in the image. Once the image is satisfactory, freeze it and then begin measurements. Start with setting the caliper on the outside of the skull, nearest to the transducer (or the mother’s abdomen). Hit “set”. Then another cursor will appear, which can be dragged to the opposite side, linking both calipers. Place that second marker just INSIDE the skull line. This connected line should be at a 90 degree angle (perpendicular) to the falx or the long axis of the head.

Figure 5. Simplified illustration demonstrating the proper caliper placement on the fetal skull to measure BPD.

BPD Can be done starting at 12 wks

Biparietal diameter (BPD) can be measured after 12 weeks (with most machines) when the cranium is developed and brain landmarks are visible, though between 12-14 weeks these may still be hard to identify clearly.

Fetal cerebellum

The fetal cerebellum is a key biometry landmark, but only because it should be avoided in the view; it should not appear in the proper plane for measuring BPD or HC. Seeing it means the view is too far posterior. While standard ultrasounds measure cerebellum width to assess gestational age, point-of-care ultrasound avoids including it. Instead, use the green dotted line (Figure 6) as the accurate plane for BPD measurement.

Figure 6. Illustration and image of the posterior brain including the cerebellum (which indicates an incorrect angle across the brain).

This image shows part of the baby’s face. If orbits, ears or the maxilla are viewed in the image, the imaging plane is incorrect for measuring the BPD or HC. The green dotted line indicates the correct plane for a BPD

Figure 7.. Illustration and image an incorrect angle across the brain, angled too far forward anteriorly, including the fetal face and cheeks).

BPDs at different gestational ages

The next section presents images of BPD done at various gestations. Although this seems redundant, it gives the learner a chance to view many times over the normal structures and caliper placements.

Images 3, 4. BPDs done at 14 and 15 weeks gestation.

The images above show brain development at 14 and 15 weeks. The left image (14 weeks) lacks clear brain landmarks, while the right image (15 weeks) shows visible thalami and choroids.

Images 5, 6. BPDs done at 16 and 18 weeks gestation.

At 16-18 weeks (above) the anatomic structures are more easily identified

Image 7. BPD at 19 weeks gestation.

Images 8, 9. BPDs at 21 weeks’ gestation

Images 8 and 9 (above) are from 22-23 weeks and at 23.6 wks where the structures are very clearly seen.

Image 10, 11. BPDs at 22 and 23.6 weeks gestation

Images 12 and 13 (below) are from 24-25 wks gestation

As gestational age increases, the fetal head enlarges and structures become more distinct. It can become necessary to adjust the focal zone to fit the head within the screen view.

Images 14, 15. BPDs at 27 and 29 weeks gestation.

Images 16, 17. BPDs at 28.6 weeks and 29.3 weeks gestation.

Image 18 (below) shows both BPD and head circumference at 30 weeks. As the baby's head now takes up much of the view, adjust the depth to resize the head to fit within the sector. Using the depth adjustment, the head will appear smaller, and therefor fit better into the image sector.

The image below (Image 17) shows the BPD and HC of a 30-week fetus, illustrating how gestational age is estimated from these measurements. If there's a slight difference between the measurement-based age (e.g., 30 weeks 4 days) and your LMP (e.g., 31 weeks 2 days), you should not change the due date; small variations are normal. Acceptable and unacceptable margins or discrepancies will be reviewed in the chapters on fetal growth.

Image 18. BPD at 30 weeks gestation

Below are two images of BPDs at 32 weeks (images 19 and 20). At this stage, including the entire head within the field of view can be challenging.

Images 19, 20. BPDs at 32.6 weeks and 32.0 weeks gestation.

At 33 weeks, although the fetal head does not fit well into the sector, BPD measurement remains feasible. If most of the fetal skull is seen clearly, the ellipse can be placed on that area, and it will automatically extrapolate to where it thinks the unseen edge of the skull is located.

Image 21. BPD at 33 weeks gestation.

Images 22. 23. BPDs at 34 and 35 weeks gestation.

Imaging a late third trimester fetal head can be challenging, as many babies begin descending into the pelvis at this stage. This makes it difficult to obtain accurate biometric measurements if the head is not optimally positioned or pelvic bones obstruct access.

Image 24. BPD at 37 weeks’ gestation.

Inaccurate BPD measurements

The following images illustrate incorrect measurements. the left image below (24), the calipers are not exactly perpendicular to the falx, and the head appears slightly asymmetric. In the right image (25), brain structures are unclear, and the head is too round to represent a valid BPD plane.

Images 25, 26. Incorrect views and measurements of the BPD.

Below is an example of a poor BPD measurement (image 27).

This image displays the falx and thalami, but facial features (cheeks) should not be seen in the correct viewing plane.

Image 27. Incorrect plane for BPD showing part of the face (cheek).

BIPARIETAL DIAMETER - SUMMARY

Location and use of consistent landmarks improves the accuracy of the biparietal measurements by confirming the correct plane of scanning.

The landmarks used to identify the correct angle for measuring are:

falx in the midline of the skull
thalamus
cavum septum pellucidum
uninterrupted calvarium
absence of cerebellum

Calipers are to be placed from the outer edge of one side of the BPD and extended to the inner edge of the opposite side.

Even with very accurate measurements, the margin of error for second trimester measuring can be up to 10 days’ difference from the true gestational age. During mid to late third trimester this measurement can have a margin of error of up to 14-21 days. This makes the measurement less helpful for determining growth concerns, but it does assist in giving a general idea of the gestational age of a woman who presents late in pregnancy for prenatal care.

For any measurement that seems to be outside of these guidelines, the woman should have a complete biometric assessment by a sonographer.

GULU Midwives Basic Obstetric POCUS

Chapter 12. Basic Biometry using Biparietal Diameter only