Objective: To develop and test a new formula for estimating weight in the fetus of < or = 1600 g.
Methods: A formula for sonographic estimation of fetal weight was produced retrospectively from 84 singleton fetuses with a birth weight of < or = 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.
Results: 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 x head circumference + 2.069 x femur length3 + 0.0232 x abdominal circumference3-6235.478 x 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%.
Conclusion: 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 < or = 1600 g.