Estimated Birth Weight Calculator NHS
This independent UK page explains estimated fetal weight (EFW) from ultrasound measurements. It does not predict the baby’s eventual birth weight from gestational age alone. Current UK maternity guidance uses ultrasound biometry and an accepted EFW formula/chart to assess fetal size and growth.
Hadlock estimated fetal weight calculator
Enter ultrasound measurements in centimetres. This uses the Hadlock HC-AC-FL equation described in UK fetal-growth guidance.
How estimated fetal weight is used
Ultrasound measurements of the baby’s head, abdomen and femur can be combined to estimate fetal weight. The result is then interpreted against an appropriate gestational-age growth chart and alongside previous measurements, Doppler findings and the wider clinical picture.
Important 2026 UK chart update
In December 2025 NHS England asked maternity units in England to stop using INTERGROWTH estimated fetal weight charts because of concerns that they could identify fewer small-for-gestational-age babies. In July 2026 RCOG clarified support for Hadlock, WHO, Fetal Medicine Foundation (FMF) and GROW EFW standards and did not recommend one of those four over all the others.
Formula
This tool uses the Hadlock equation based on head circumference (HC), abdominal circumference (AC) and femur length (FL): log10(EFW) = 1.326 − 0.00326(AC×FL) + 0.0107(HC) + 0.0438(AC) + 0.158(FL). Measurements are entered in centimetres and EFW is returned in grams.
Worked example
If an ultrasound report gives HC 32.0 cm, AC 30.0 cm and FL 6.5 cm, the equation gives an estimated fetal weight of about 2.3 kg. The clinical meaning depends on gestational age and the growth chart used by the maternity service.
What does a low estimated fetal weight mean?
RCOG commonly describes a fetus as small for gestational age when EFW or relevant biometric measurements are below the 10th centile. Being small does not automatically mean fetal growth restriction; clinicians assess growth pattern and other evidence before making that distinction.
Limitations
EFW is an ultrasound estimate, not an exact fetal or future birth weight. Measurement error, fetal position, gestation and the formula/chart used can affect the result. Do not use a home-entered estimate to decide whether growth is normal or to make decisions about monitoring or timing of birth.
Authoritative Sources
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FAQs
Is estimated fetal weight the same as birth weight?
No. EFW estimates fetal weight at the time of an ultrasound; it cannot know the baby’s exact weight or eventual birth weight.
Which measurements does this calculator use?
It uses ultrasound head circumference, abdominal circumference and femur length in the Hadlock HC-AC-FL equation.
Does a result below the 10th centile mean the baby is unhealthy?
Not necessarily. Some babies are constitutionally small. Clinical assessment is needed to distinguish a small baby from fetal growth restriction.
Which growth chart should be used in the UK in 2026?
RCOG’s July 2026 update supports Hadlock, WHO, FMF and GROW EFW standards and does not specifically recommend one of those four over another. NHS England has asked English maternity units to stop using INTERGROWTH EFW charts.
Disclaimer: This educational calculator cannot replace a sonographer, midwife or obstetric assessment. Contact your maternity team about concerns regarding fetal growth or movements.