Fetal biometry is the ultrasound measurement of standard fetal landmarks used to assign gestational age, track growth, and estimate fetal weight. At the scanner the task is concrete: obtain the correct plane, place calipers to match the chart in use, and decide whether the result dates the pregnancy, describes growth, or needs to be repeated. This article reviews biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL), plus related diameters such as occipitofrontal diameter (OFD) and transverse abdominal diameter (TAD), with emphasis on technique, dating versus growth, and pitfalls that skew gestational age or estimated fetal weight (EFW).
Fetal Biometry Planes: BPD, HC, AC, and FL
After the first trimester, the core biometric set is BPD, HC, AC, and FL. Many protocols also store OFD so HC can be calculated or a cephalic index checked, and some systems compute AC from TAD and the anteroposterior abdominal diameter. Crown-rump length remains the preferred dating measurement when a valid first-trimester CRL is available. Later measurements then describe size relative to those dates rather than rewriting them.
Lead with the plane, not the number. A careful measurement in the wrong section will be precisely wrong. Freeze when landmarks are complete and the structure is as symmetric as fetal position allows, then place calipers.
BPD in fetal biometry
BPD is taken on an axial transthalamic head view. You should see the cavum septi pellucidi anteriorly, the thalami on either side of a continuous midline falx, and equal hemispheres. The cerebellum and orbits must be out of plane. Caliper convention is not universal. Many North American workflows and Hadlock-linked packages use the outer edge of the near calvarium to the inner edge of the far calvarium (leading edge to leading edge). Other charts use outer to outer. Match the method to the reference. BPD in fetal biometry is vulnerable to head shape. Dolichocephaly and brachycephaly move BPD off the mean even when HC is appropriate, so do not date or estimate weight from BPD alone when the head is elongated or rounded. In late pregnancy, molding can further degrade BPD; HC is the more reliable head size parameter in that setting.
HC and OFD
HC is the more stable head parameter when molding, presentation, or an unusual cephalic index is present, and it is widely preferred for second-trimester dating if CRL was never obtained. Measure HC in the same transthalamic plane, using an ellipse along the outer skull bone and excluding skin. If your chart was built from diameters, calculate HC from BPD and OFD rather than mixing methods. OFD is outer to outer from the mid-frontal to mid-occipital bone. A commonly cited cephalic index (BPD divided by OFD) falls roughly in the 0.70 to 0.85 range; use your laboratory's limits. When the index is outside that range, lean on HC and AC, not BPD.
AC and TAD
AC contributes the most to EFW and is the biometric value most often abnormal in fetal growth restriction and in macrosomia. The plane is a true axial section through the abdomen that includes the stomach and the intrahepatic umbilical vein at its junction with the portal sinus, without kidneys, heart, or lungs. Ribs should appear as short symmetric segments. Place an ellipse on the outer skin line, including subcutaneous fat and excluding the cord. TAD is the skin-to-skin transverse diameter in that plane. A compressed, oblique, or too-caudal abdomen systematically underestimates AC and therefore EFW.
FL: the femur in fetal biometry
FL is the length of the ossified femoral diaphysis, not the cartilaginous ends. Align the shaft perpendicular to the beam with both blunt metaphyseal ends on the image. The femur closer to the transducer is often the cleaner measurement. Place calipers at the ends of ossified bone only. Do not include the distal femoral epiphysis or a thin distal spike. Confirm you have not measured the humerus. An off-axis femur looks short. If FL is discordant with head and abdomen, repeat the measurement, survey the other long bones, and consider dates, family stature, and skeletal dysplasia in the differential. Structured review of limb findings is covered in IAME Case Studies: Fetal Extremities.
How to Use Fetal Biometry for Dating Versus Growth
Assign dates once. Prefer a high-quality first-trimester CRL when it is available, then interpret all later biometry as growth. If the first ultrasound is in the second trimester, HC (sometimes combined with BPD, depending on the guideline set you follow) is commonly used to establish gestational age. Third-trimester biometry should rarely change the due date, because both normal biologic variation and true growth disorders are large enough to mislead.
Specialty guidance describes discrepancy windows in which ultrasound dates replace the last menstrual period. Those windows widen as pregnancy advances, and details differ among organizations and over time. Use your current ACOG, AIUM, or institutional policy rather than a memorized cutoff. For first-trimester technique that underpins later growth interpretation, see IAME's course on advanced fetal ultrasound for early first trimester assessment.
Once dates are fixed, report size as measurements, EFW, and percentiles on a single, consistent chart. Interval change matters as much as a one-time percentile. A falling AC or EFW percentile, an AC that lags the head, or an EFW below the threshold your practice uses for small-for-gestational-age surveillance should trigger the follow-up pathway you already have in place, not an automatic redating. An appropriate HC with a small AC suggests an asymmetric pattern often discussed with placental insufficiency. A globally small set of measurements raises incorrect dates, a constitutionally small fetus, and an early symmetric insult in the differential. These patterns support clinical judgment; they do not replace it.
Caliper Placement Checklist
Use this as a freeze-frame audit before you store the image:
- Head: transthalamic plane, cavum visible, no cerebellum, hemispheres equal, calipers on bone using the chart's outer-inner or outer-outer rule, ellipse on outer skull for HC.
- OFD: same plane as BPD, outer to outer, frontal to occipital.
- Abdomen: stomach plus portal sinus, no kidneys or heart, round section, ellipse on outer skin, cord out of the measurement.
- Femur: full ossified shaft, beam perpendicular, calipers on bone ends only, correct bone labeled.
- Same reference chart for every measurement and for EFW, including Hadlock versus other datasets.
- Do not average a good plane with a bad one to split the difference toward expected dates.
Pitfalls That Skew Gestational Age or Estimated Fetal Weight
- Wrong head level. Too caudal includes the posterior fossa and distorts BPD and HC. Too cranial loses the cavum and thalami.
- Head shape ignored. Using BPD when the cephalic index is abnormal biases dating and any Hadlock formula that includes BPD.
- Mismatched caliper convention. Outer-to-inner BPD on an outer-to-outer chart, or the reverse, shifts gestational age by a clinically meaningful amount, especially in mid-pregnancy.
- HC ellipse on skin. Including scalp overestimates HC.
- Oblique or caudal AC. Kidneys in the image, an ovoid compressed abdomen, or an ellipse that clips the fat underestimates AC, the main driver of EFW.
- Cord in the AC. Including the umbilical cord overestimates AC.
- Short femur artifacts. Off-axis shaft, far-field dropout, measuring cartilage, or labeling a humerus as FL all underestimate FL.
- Confirmation bias. Repeating only the measurement that disagrees with dates until it agrees can hide growth restriction or incorrect dates.
- Redating at every visit. This can mask fetal growth restriction or make a well-grown fetus look large.
- Mixing charts. A Hadlock EFW plotted on a different population standard is not a valid percentile.
Image quality still matters. High BMI, oligohydramnios, late gestation, and an engaged head all degrade planes. Document when a measurement is technically limited rather than forcing calipers onto a nonstandard image.
EFW, Hadlock Formulas, and Percentiles
Most reporting packages calculate EFW from a regression that includes some combination of HC, AC, FL, and BPD. Hadlock formulas remain common in the United States. Combinations that include AC, FL, and HC are widely used; adding BPD helps only when the head measurement is valid. Because AC carries so much weight in these equations, a technically poor AC becomes a larger error in EFW.
EFW is an estimate. A commonly cited typical error is on the order of 10 to 15 percent, and disagreement is often larger at the extremes of weight and near term. Do not treat a single EFW as a birth weight. Interpret percentiles with the same dataset used to compute EFW. The NICHD Fetal Growth Study, INTERGROWTH, WHO, and older Hadlock curves will not give identical cutoffs. State the chart on the report so the next reader does not mix standards.
What a Growth Scan Is and When It Is Done
A growth scan is an ultrasound, usually in the third trimester, performed to measure fetal size after gestational age has already been assigned. It typically includes the standard biometric set, amniotic fluid, and, when indicated, Doppler. It is not the same examination as the mid-trimester anatomy survey, although that survey also records fetal biometry.
Timing depends on indication and local protocol. Common reasons include a size-dates discrepancy, hypertensive disease, diabetes, prior growth-restricted or stillborn infant, twins, and other maternal or fetal risk factors. Some groups offer a routine third-trimester growth examination; others reserve it for risk. There is no single week that is mandatory for every pregnancy. Anatomy scans are commonly performed around 18 to 22 weeks. Indicated growth scans are often scheduled in the late second or third trimester and repeated at intervals set by the clinical problem. Confirm current guidance with your practice and referring clinicians.
How to Read Pregnancy Ultrasound Abbreviations
Reports compress a full biometric worksheet into a short string of abbreviations. The ones that appear with fetal biometry include:
- CRL: crown-rump length, first-trimester dating.
- BPD: biparietal diameter.
- OFD: occipitofrontal diameter.
- HC: head circumference.
- AC: abdominal circumference.
- TAD / APAD: transverse and anteroposterior abdominal diameters.
- FL: femur length; HL is humerus length if it is reported.
- CI: cephalic index (BPD/OFD).
- EFW: estimated fetal weight, often from a Hadlock-type formula.
- GA, LMP, EDD: gestational age, last menstrual period, estimated due date.
- SGA / LGA / FGR: small for gestational age, large for gestational age, fetal growth restriction (definitions vary by society).
Read the numbers with the plane quality, the chart name, and whether dates were assigned in the first trimester. A percentile without that context is easy to over-interpret.
Correct planes and honest caliper placement matter more than any single published normal millimeter value. Use fetal biometry to lock dates early, then to describe growth with a consistent chart, and repeat any measurement that does not show the landmarks. IAME offers accredited ultrasound CME courses on fetal imaging technique and interpretation for clinicians who want a structured review of scanning protocol and measurements.
Frequently Asked Questions
What is normal HC at 21 weeks?
There is no single normal HC at 21 weeks, because reference charts and populations differ. On many widely used curves, mean HC at 21 weeks is near 18 to 19 cm, with a spread of several centimeters across the central percentiles. Always compare HC with the same chart and caliper method used to acquire it, and interpret it together with BPD, AC, FL, and dates rather than as an isolated cutoff.
What is the femur in fetal biometry?
The femur measurement (FL) is the ossified femoral diaphysis on a long-axis view, with calipers at the bony ends and without the cartilaginous epiphysis. It contributes to gestational age assessment when dates are still being assigned and to EFW and interval growth later in pregnancy. A short FL should be confirmed in plane, compared with the other long bones, and interpreted in light of dates, parental stature, and the rest of the anatomic survey.
What is a growth scan in pregnancy and when is it done?
A growth scan is a biometric ultrasound, most often in the third trimester, used to assess fetal size after gestational age is already established. It is commonly performed for size-dates discrepancy, maternal disease, multiple gestation, or a prior growth-related outcome, and in some practices as a routine later scan. Timing and Doppler add-ons follow indication and local protocol. The mid-trimester anatomy exam also includes biometry but is not itself a dedicated growth study.
How to read pregnancy ultrasound abbreviations?
Start with the four core measurements: BPD (biparietal diameter), HC (head circumference), AC (abdominal circumference), and FL (femur length). OFD is occipitofrontal diameter, TAD is transverse abdominal diameter, CRL is the first-trimester crown-rump length, and EFW is estimated fetal weight. Percentiles and SGA or LGA labels are chart-dependent, so read them against the stated reference and against whether dates came from first-trimester CRL.
