Fetal Biometry Explained: BPD, HC, AC and FL
Fetal biometry is the set of standard ultrasound measurements used to assess fetal size and growth. Four measurements are routinely used after the first trimester: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur length (FL). Together they are used to estimate fetal weight.
Biparietal diameter (BPD)
BPD is the width of the fetal head, measured across the two parietal bones in a transverse section of the head. The standard plane is the transthalamic plane, which shows the thalami, the cavum septi pellucidi and the midline echo, with the skull outline symmetrical and oval.
BPD can be measured in two ways: outer-to-inner (from the outer edge of the near parietal bone to the inner edge of the far one) or outer-to-outer. The two methods give slightly different values, so the method must match the reference chart. The WHO fetal growth study used the outer-to-inner method.
BPD is affected by head shape. A long, narrow head (dolichocephaly, common in breech presentation or with reduced liquor) gives a low BPD even when the head is normal-sized. In that case HC is the more reliable head measurement.
Head circumference (HC)
HC is measured in the same plane as BPD, by placing an ellipse around the outer border of the skull. Because it is a circumference, HC is less affected by head shape than BPD. It is the preferred head measurement for growth assessment and is one of the inputs of the commonly used Hadlock EFW formulas.
A small HC on serial scans can be part of symmetrical smallness. A head much smaller than the other parameters raises the question of microcephaly, which needs specific assessment and is not decided by a growth chart alone.
Abdominal circumference (AC)
AC is measured in a transverse section of the fetal abdomen at the level of the stomach and the umbilical vein in the liver, with the section as close to circular as possible. The ellipse is placed on the skin surface.
AC reflects the size of the fetal liver and the subcutaneous fat, which depend on fetal nutrition. For that reason AC is the most sensitive single measurement for growth restriction and for excessive growth. In placental insufficiency, AC is often the first parameter to fall behind, while the head is relatively spared. AC is also the hardest measurement to take reproducibly, because the abdomen changes shape with fetal breathing, position and probe pressure.
Femur length (FL)
FL is the length of the ossified femoral shaft (diaphysis), measured with the whole shaft visible and the beam as close to perpendicular to the bone as possible. The ends of the shaft are measured. The distal femoral epiphysis is not included.
A slightly short FL in isolation is common and often reflects the measurement angle or the family's build. A markedly short femur, or short long bones together with other findings, needs further assessment.
How the WHO study measured them
The centiles on this site come from the WHO Fetal Growth Charts study. Its methods describe the measurement standard used to build the reference:
| Parameter | WHO study method |
|---|---|
| BPD | Outer–inner distance of the parietal bones, in a cross-sectional view of the head |
| HC | From the same image as BPD, ellipse around the outer skull perimeter |
| AC | Transverse section of the fetal abdomen, as close to circular as possible |
| FL | Image of the full femoral shaft, in a plane close to 90° to the ultrasound beam |
| EFW | Hadlock's formula using HC, AC and FL |
When you plot on WHO charts, it is good practice to measure with the same technique, especially BPD outer-to-inner.
Getting reliable measurements
- Use the correct plane. Most measurement error comes from an oblique or wrong plane, not from caliper placement.
- Magnify the image so the structure fills most of the screen before placing calipers.
- Measure more than once. Many units take the average of two or three measurements for each parameter.
- Record the GA correctly. The tool needs POG in completed weeks and days (for example 28 weeks + 3 days).
- Check units. BPD, HC, AC and FL are entered in millimetres and EFW in grams. The tool warns when a value looks far from the expected range.
References
- Kiserud T, Piaggio G, Carroli G, et al. The World Health Organization Fetal Growth Charts: a multinational longitudinal study of ultrasound biometric measurements and estimated fetal weight. PLOS Med. 2017;14(1):e1002220. doi:10.1371/journal.pmed.1002220
- Salomon LJ, Alfirevic Z, Da Silva Costa F, et al. ISUOG Practice Guidelines: ultrasound assessment of fetal biometry and growth. Ultrasound Obstet Gynecol. 2019;53(6):715–723.