Baby Size Calculator
Estimates fetal weight and size using gestational age alone or ultrasound measurements (BPD, FL, AC). Used by parents and clinicians to track growth against standard developmental charts.
Last updated: May 2026
About this calculator
Two estimation methods are available. Without ultrasound data, the calculator uses the Hadlock (1991) gestational-age weight standard: ln(weight_g) = 0.578 + 0.332×W − 0.00354×W², where W is gestational age in weeks. This is a natural-log model — weight = e^(…) — and returns the population median (roughly 50th-percentile) fetal weight for that age. When you enter ultrasound biometry, the Hadlock (1985) three-measurement formula is applied instead: log₁₀(weight_g) = 1.335 − 0.0034×AC×FL + 0.0316×BPD + 0.0457×AC + 0.1623×FL, so weight = 10^(…). BPD (biparietal diameter), FL (femur length), and AC (abdominal circumference) are entered in millimeters and converted internally to centimeters — the unit Hadlock's regression was derived in. Both outputs are in grams. Leave the three ultrasound fields blank to use the gestational-age method alone; fill all three to switch to the biometry estimate. Gestational age is expressed as weeks plus fractional days (days ÷ 7).
How to use
Suppose you are 28 weeks and 3 days pregnant and your ultrasound shows BPD = 71 mm, FL = 53 mm, and AC = 245 mm. The calculator converts these to centimeters (7.1, 5.3, 24.5) — the unit Hadlock's regression uses — and applies the 1985 formula: log₁₀(weight) = 1.335 − (0.0034 × 24.5 × 5.3) + (0.0316 × 7.1) + (0.0457 × 24.5) + (0.1623 × 5.3) = 1.335 − 0.441 + 0.224 + 1.120 + 0.860 = 3.098. Weight = 10^3.098 ≈ 1,252 g. If you have no ultrasound numbers, leave BPD, FL, and AC blank: the calculator then uses the Hadlock (1991) gestational-age median, e^(0.578 + 0.332 × 28.43 − 0.00354 × 28.43²) ≈ 1,281 g for 28 weeks 3 days. Compare your result against a percentile chart with your provider — a single estimate carries a ±15–20% margin.
Frequently asked questions
How accurate is fetal weight estimation from ultrasound measurements?
Hadlock-based ultrasound estimates of fetal weight carry an inherent error of approximately ±15–20% even under ideal conditions. Accuracy decreases as gestational age increases because adipose tissue deposition in late pregnancy is harder to measure sonographically. Multiple measurements taken by the same sonographer on the same machine improve consistency. Clinicians use these estimates alongside growth trends over time, not as single absolute values.
What do BPD, FL, and AC mean in a baby size ultrasound measurement?
BPD (biparietal diameter) is the width of the fetal skull measured from one parietal bone to the other — an indicator of head growth. FL (femur length) measures the longest fetal bone and serves as a proxy for skeletal maturity. AC (abdominal circumference) reflects the size of the fetal abdomen and correlates strongly with liver size and subcutaneous fat, making it the most sensitive single measurement for detecting intrauterine growth restriction. Together these three measurements feed into validated formulas like Hadlock to estimate fetal weight.
When should I be concerned about my baby's size according to the calculator?
Estimated fetal weight below the 10th percentile for gestational age is defined as small for gestational age (SGA) and warrants further evaluation for intrauterine growth restriction. Weight above the 90th percentile is large for gestational age (LGA), which can be associated with gestational diabetes. A single measurement is less informative than serial ultrasounds tracked over 2–4 weeks. Always discuss results with your obstetrician or midwife rather than acting on calculator output alone.