Gestational Age from Fetal Biometry Calculator (BPD, HC, AC, FL — Hadlock)
Estimate gestational age from second- and third-trimester ultrasound measurements — BPD, head circumference, abdominal circumference and femur length — with the Hadlock formulas and an estimated due date.
How it works
Measurements in cm. BPD: 9.54 + 1.482·BPD + 0.1676·BPD² · HC: 8.96 + 0.540·HC + 0.0003·HC³ · AC: 8.14 + 0.753·AC + 0.0036·AC² · FL: 10.35 + 2.460·FL + 0.170·FL² · all four: 10.85 + 0.060·HC·FL + 0.6700·BPD + 0.1680·AC (weeks)
Hadlock and colleagues (1984) derived regression equations that convert standard fetal measurements into menstrual (gestational) age, and they remain the default in most ultrasound software. Combining several parameters reduces error, so when all four are entered the calculator uses Hadlock's 4-parameter formula; otherwise it averages the single-parameter ages. Dating is most accurate early: crown–rump length before 14 weeks is preferred, and in the second trimester biometry dates within about ±7–10 days, widening to ±3 weeks near term.
Worked example
A scan shows BPD 47 mm, HC 175 mm, AC 150 mm and FL 33 mm. Single-parameter ages are 20 weeks 1 day (BPD), 20 weeks 0 days (HC), 20 weeks 2 days (AC) and 20 weeks 2 days (FL). Hadlock 4-parameter: 10.85 + 0.06 × 17.5 × 3.3 + 0.67 × 4.7 + 0.168 × 15 = 19.99 weeks = 20 weeks 0 days.
Sources: Hadlock FP et al. (1984) Estimating fetal age: computer-assisted analysis of multiple fetal growth parameters. Radiology 152:497–501 · ACOG Committee Opinion 700 — Methods for estimating the due date · Perinatology.com — Hadlock equations
Frequently asked questions
What do BPD, HC, AC and FL mean?
BPD is the biparietal diameter (width of the head), HC the head circumference, AC the abdominal circumference and FL the femur (thigh bone) length. Together they are called fetal biometry and are measured at the anatomy scan and growth scans.
Should the due date change after a later scan?
Usually not. ACOG recommends dating by the first-trimester scan (or last period if consistent) and changing the due date only when a later scan differs by more than set limits — for example more than 10 days at 16–21 weeks. Later differences more often reflect growth than wrong dates.
Why do the measurements give different ages?
Each part of the body grows at its own rate and babies vary in shape. A larger head or a smaller abdomen is common and normal; your sonographer interprets the pattern, including percentiles, which this calculator does not assess.
Do I enter millimetres or centimetres?
Millimetres, as most reports show. If your report shows centimetres, multiply by 10 (4.7 cm = 47 mm).
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This calculator is for education only and does not replace your obstetric care team. Ultrasound measurements, their interpretation and any dating or diagnosis must come from a qualified sonographer or clinician.