Work out foetal weight estimate instantly with clear inputs, formula shown and shareable results.
Applies Johnson's formula, which multiplies the symphysis-fundal height minus an engagement constant by 155 to estimate foetal weight.
Johnson's formula
Estimated weight (g) = (fundal height in cm - 12 if the head is not engaged, or 11 if engaged) x 155
Educational estimate only. Not a substitute for professional clinical judgement, and must not be used to make treatment or dosing decisions.
It is a clinical estimate with wide error margins; ultrasound biometry using Hadlock formulae is far more precise.
Yes. Obesity, fibroids and polyhydramnios all distort fundal height and therefore the estimate.