gestational age BPD second trimester fetal biometry

Fetal Biometry in 2nd and 3rd Trimesters: BPD, HC, AC and FL

After 14 weeks, CRL loses accuracy and gestational dating uses a set of biometric measurements: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur length (FL).

Medical disclaimer: this calculation is educational and does not replace physician evaluation. Biometric measurements have inherent margin of error and must always be interpreted in clinical context.

Biometric measurement type

Crown-Rump Length (CRL)in mmUltrasound exam dateOptional — to calculate EDD

How to use this calculator

Select the biometric measurement type from your ultrasound report, enter the value in millimeters, and optionally enter the exam date to get the estimated EDD.

CRL1st trimester · 6–14 wks

BPD2nd/3rd trim. · 12–40 wks

HC2nd/3rd trim. · 12–40 wks

AC2nd/3rd trim. · 14–40 wks

FL2nd/3rd trim. · 14–40 wks

The margin of error for biometry-based estimates is ± 0.5 to 2.5 weeks, depending on the measurement and gestational age. CRL in the 1st trimester is the most precise parameter.

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The role of each measurement

BPD measures skull width and is used to estimate GA and assess cephalic growth. HC complements BPD, being more sensitive for microcephaly. AC is the best marker of intrauterine growth restriction (IUGR). FL assesses longitudinal bone growth.
The composite estimate (all measurements combined) is more accurate than any single measurement. Modern ultrasound machines automatically calculate composite GA and estimated fetal weight.

Second-trimester biometry examples

BPD of 50 mm

Input: DBP: 50 mm
Expected output: ~20 semanas

Coincides with the mid-pregnancy anatomy scan.

AC of 268 mm

Input: CA: 268 mm
Expected output: ~30 semanas

AC below p10 for this GA suggests IUGR investigation.

Full tool FAQ

Which measurement is most accurate for estimating GA?

CRL (Crown-Rump Length) in the 1st trimester (weeks 6–14) is the most precise parameter, with a margin of error of only ± 3–5 days. After week 14, composite biometry (BPD + HC + AC + FL) tends to be more accurate than any single measurement. Precision decreases progressively throughout gestation.

Can ultrasound 'correct' the due date calculated from LMP?

Why does the margin of error increase later in pregnancy?

Can this calculator be used for twin pregnancies?

What is the clinical significance of each measurement?

Which formulas does the calculator use?

Can I combine more than one measurement for better accuracy?

What should I do if the GA estimated here differs from my ultrasound report?

Frequently asked questions

What is a percentile in the context of fetal biometry?

Percentile indicates where the fetal measurement sits relative to a reference population at the same GA. Percentile 50 is the median. Values below p10 or above p90 require medical evaluation to rule out abnormal growth.