Fetal Weight Estimator

This calculator looks up the average weight and length for a gestational week. It is the population curve, not a measurement of your baby. If you have had a scan, the number your sonographer gave you came from a formula like Hadlock’s below, using measurements of the head, abdomen and femur. Type that number in as well and the page shows which percentile band it falls in.

Where the numbers come from

From 20 weeks the weights are the Hadlock 1991 in-utero standard (Radiology, volume 181), built from 392 pregnancies with certain dates and still the default in many ultrasound machines. Its 50th percentile follows one equation, ln(weight in grams) = 0.578 + 0.332 × weeks − 0.00354 × weeks², and the printed 10th and 90th percentiles sit 17% below and above it. This page uses the equation, so it matches the printed table to within a gram and can handle weeks plus days.
Before 20 weeks the figures are typical anatomical weights from pregnancy references. Hadlock’s curve runs high that early, so it is not used there.

Look at how wide those ranges are. At 36 weeks the healthy tenth percentile and the healthy ninetieth are almost a kilogram apart. Being off the average is the normal condition, and the percentile band matters far more than the midpoint does.

Hadlock’s model stops at 41 weeks. At 42 the page shows the 41-week figures and says so.

Your scan report may not use Hadlock at all. Many units now plot against the WHO fetal growth charts, INTERGROWTH-21st, or in the UK the customised GROW charts, which adjust for the mother’s height, weight and ethnicity. The same estimate can land on a different percentile on each, so ask which chart your report uses before comparing.

What the percentile bands mean

Below the 10th percentile is small for gestational age. In the US, an estimated weight or abdominal circumference below the 10th percentile is how the Society for Maternal-Fetal Medicine defines fetal growth restriction (2020), and below the 3rd is treated as severe. Above the 90th is large for gestational age. Both lead to more scans and closer checks, not straight to delivery. Plenty of babies below the 10th are simply small and healthy, which is exactly what the follow-up scans are for.

A note about the length figures

Fetal length is measured two different ways depending on how far along you are, and the changeover produces a jump that alarms people. Before 20 weeks it is crown to rump, because the legs are curled up tight against the body and cannot be measured reliably. From 20 weeks it is crown to heel, with the legs extended. That is why the table steps from about 15 cm at week 19 to about 26 cm at week 20. Nothing grew 11 cm in a week; the ruler changed. The weight steps a little at 20 weeks too, where the source changes to Hadlock.

How a scan actually estimates weight

Hadlock (1985), the four-parameter version used in most ultrasound machines:

log₁₀(EFW) = 1.3596 − 0.00386(AC×FL) + 0.0064(HC) + 0.00061(BPD×AC) + 0.0424(AC) + 0.174(FL)

Two-parameter Hadlock (AC + FL), used when the head is hard to measure:

log₁₀(EFW) = 1.304 + 0.05281(AC) + 0.1938(FL) − 0.004(AC×FL)

What each variable means. All measurements go into these formulas in centimetres, which is the single most common mistake made with them: enter millimetres and the answer comes out as nonsense.

• AC (Abdominal Circumference): measured at the level of the fetal liver and stomach. The single best predictor of fetal size and nutritional status.
• FL (Femur Length): the length of the thigh bone. Correlates with overall skeletal growth.
• HC (Head Circumference): circumference of the fetal skull. Reflects brain growth.
• BPD (Biparietal Diameter): the width of the skull from side to side.
• EFW (Estimated Fetal Weight): the result, in grams.

Worked example, two-parameter, at 32 weeks: AC = 28.3 cm, FL = 6.2 cm.

log₁₀(EFW) = 1.304 + 0.05281×28.3 + 0.1938×6.2 − 0.004×28.3×6.2
= 1.304 + 1.4945 + 1.2016 − 0.7018 = 3.2983
EFW = 10^3.2983 = 1,987 g, which is 102% of the 1,953 g median for 32 weeks and comfortably inside the 1,621 to 2,285 g band. Enter 32 weeks and 1987 above to see it.

Important: an ultrasound EFW carries a margin of error of ±10–20%. At term that is potentially half a kilogram either way, which is why a scan predicting a large baby is a poor reason on its own to schedule an early delivery. It is a screening estimate, not a measurement, and the trend across scans tells you more than any one of them.