📏 Fetal Biometry–Gestational Age (Hadlock)
Derives the gestational age implied by each fetal biometric parameter and a composite ultrasound GA (Hadlock 1984), with an estimated fetal weight, to flag growth deviation against clinical dating.
Fetal Biometry–Gestational Age (Hadlock)
Biparietal diameter BPD (mm)
Head circumference HC (mm)
Abdominal circumference AC (mm)
Femur length FL (mm)
Clinical GA (optional, by LMP / first-trimester CRL) (wk)
When to use
Enter BPD/HC/AC/FL in millimeters (and optional clinical GA); the tool converts to cm internally and reports per-parameter GA, composite GA, EFW, and any asymmetry or dating discrepancy.
How it works
Per-parameter GA from Hadlock 1984 quadratics/cubics (e.g. AC-GA = 8.14 + 0.753·AC + 0.0036·AC²). Composite GA prefers the 4-parameter formula 10.85 + 0.060·HC·FL + 0.67·BPD + 0.168·AC. EFW by the available-parameter Hadlock regression.
Key points
- An AC-implied GA lagging the head/femur by ≥ 2 weeks suggests asymmetric (placental) growth restriction; exceeding it by ≥ 2 weeks suggests macrosomia — both are flagged. (original synthesis · not guideline verbatim)
- An established GA from first-trimester CRL or reliable LMP is not redated by third-trimester biometry; later measurements assess growth, not dates.
- Composite GA is most accurate in the second trimester; third-trimester biometric dating carries a wider error (often ±2–3 weeks).
References
- Hadlock FP, et al. Estimating fetal age: computer-assisted analysis of multiple fetal growth parameters. Radiology. 1984;152(2):497-501.
- ACOG/AIUM/SMFM. Methods for Estimating the Due Date. Committee Opinion 700. Obstet Gynecol. 2017;129(5):e150-e154.
Decision support for licensed clinicians only; not a substitute for clinical judgement, diagnosis or local protocols.