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, and the two answer different questions. If you have had a scan, the number your sonographer gave you came from the Hadlock formula described below, using measurements of the head, abdomen and femur. Use this page for “is my baby roughly the size a baby is at 30 weeks”, and your scan report for anything else.

Average fetal weight by gestational age:

Look at how wide those ranges are. At 36 weeks the healthy tenth percentile and the healthy ninetieth are 2,100g and 3,200g, a difference of more than a kilogram. Being off the average is the normal condition, and the percentile band matters far more than the midpoint does.

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.

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: 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 sits comfortably inside the 1,400–2,100 g band for 32 weeks.

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.