17.1 Standard Biometry: BPD, HC, AC, FL & Growth Assessment

Standard fetal biometry is the quantitative backbone of every second- and third-trimester sonogram. Four core measurements — the biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL) — are combined to estimate gestational age, calculate estimated fetal weight (EFW), and screen for abnormal growth. The ARRT exam expects you to know the exact anatomic landmarks for each plane, because a measurement taken in the wrong plane is the single most common source of biometric error.

Head Biometry: BPD and HC

The BPD and HC are obtained in the same axial (transaxial) plane through the fetal head. The correct plane is identified by these landmarks:

• A smooth, symmetric, oval calvarium with a continuous hyperechoic rim
• The cavum septi pellucidi (CSP) anteriorly, appearing as a small fluid-filled box between the frontal horns
• The paired thalami in the midbrain, with the slit-like third ventricle between them on the midline
• No cerebellum in the image — if the cerebellum is visible, the plane is too low (that is the transcerebellar plane used for the cisterna magna, not for BPD)

The BPD is measured at the widest portion of the skull, perpendicular to the midline falx, from the outer edge of the near (proximal) calvarium to the inner edge of the far (distal) calvarium — the classic outer-to-inner technique. The HC is traced (or calculated from the BPD and occipitofrontal diameter) around the outer table of the calvarium at the same level, excluding the scalp. Of the head measurements, the HC is the most reproducible and is least affected by head-shape variation.

Cephalic Index

Because head shape varies, the cephalic index (CI) checks whether the BPD alone is a trustworthy age predictor:

When the CI is abnormal, the sonographer should rely on the HC rather than the BPD for age and growth equations.

Abdominal Circumference

The AC is the single most important measurement for detecting growth disorders because it reflects fetal liver size and subcutaneous fat. The correct transverse (axial) plane of the abdomen shows:

• The stomach bubble in the left upper quadrant
• The umbilical vein/portal sinus coursing in a short J-shape within the liver (a long vein segment means the plane is too oblique)
• The spine at the 3 o'clock or 9 o'clock position, which guarantees a true axial cut
• A round (not elliptical) abdominal outline

The plane must be above the kidneys and the cord insertion — both of which falsely enlarge the circumference — and below the heart. The AC is traced around the outer skin line or calculated from perpendicular AP and transverse diameters.

Femur Length

The FL is measured along the long axis of the femoral shaft with the beam perpendicular to the bone. Only the ossified diaphysis is included, from one blunt end to the other; the cartilaginous epiphyses and the distal femoral epiphysis (a secondary ossification center appearing after ~32 weeks) are excluded. A bright distal reflection at the far end (the "distal point") should not be included either. FL is the most resistant of the four parameters to head- or body-shape distortion, but it shortens in skeletal dysplasias and may be mildly short as a soft marker for trisomy 21.

Estimated Fetal Weight and Dating

Hadlock formulas estimate fetal weight by combining AC with head and femur data using logarithmic equations — for example, log₁₀(EFW) from BPD, AC, and FL, or from HC, AC, and FL. Because errors compound across parameters, EFW carries an inherent accuracy of only about ±15% (roughly two standard deviations) in the third trimester, and it should be reported with a percentile, not as a precise number.

Biometric dating accuracy declines as pregnancy advances because individual growth variation widens:

• Second trimester (14–27 weeks): approximately ±1–2 weeks
• Third trimester (≥28 weeks): approximately ±2–3 weeks

A composite age (averaging all parameters) improves precision, but pregnancies should be dated by the earliest reliable scan; a third-trimester examination should never be used to redate a well-dated pregnancy.

Growth Assessment and Ratios

Fetal growth restriction (FGR) is screened sonographically when the AC or the EFW falls below the 10th percentile for gestational age; macrosomia is suspected above the 90th percentile (EFW > 4,500 g at term). Symmetric (type I) FGR reduces all parameters proportionally and suggests an early insult (aneuploidy, infection), while asymmetric (type II) FGR — the more common uteroplacental pattern — spares the head (brain-sparing) while the AC and weight lag. Two ratios support this assessment: the HC/AC ratio, which normally falls from about 1.18 early in the second trimester toward 1.0 near term (an elevated HC/AC signals asymmetric FGR), and the FL/AC ratio, which stays nearly constant at about 22% (normal 20–24%) after 22 weeks and is gestational-age independent. Document cardiac activity, fetal number, and presentation alongside biometry on every study.