A new formula for calculating weight in the fetus of ≤ 1600 g




To develop and test a new formula for estimating weight in the fetus of ≤ 1600 g.


A formula for sonographic estimation of fetal weight was produced retrospectively from 84 singleton fetuses with a birth weight of ≤ 1600 g, examined sonographically within 1 week before delivery. Exclusion criteria were multiple pregnancy, intrauterine death and major structural or chromosomal anomalies. The new formula was then compared prospectively in an evaluation group of fetuses (n = 87) with six currently available equations for estimating weight in the preterm fetus.


Stepwise regression analysis with gestational age (in days) and fetal biometric parameters was employed to yield the best-fit formula for predicting fetal weight at birth. The new formula (estimated fetal weight = 5381.193 + 150.324 × headcircumference + 2.069 × femurlength3 + 0.0232 × abdo-minalcircumference3—6235.478 × log(head circumference)) proved to be superior to established equations. The lowest mean ± SD absolute error was 66.2 ± 59 g and the lowest mean absolute percentage error was 7.1 ± 5.9% SD when studied prospectively in the evaluation group. With the new formula, 48.3% of estimates fell within ±5% of the actual weight at birth, 73.6% fell within ±10%, 90.8% fell within ±15% and 95.4% fell within ±20%.


Our new formula is relatively easy to use and needs no adjustment to weight centiles or to fetal lie. It allows reliable weight estimation in the fetus ≤ 1600 g. Copyright © 2004 ISUOG. Published by John Wiley & Sons, Ltd.