Prediction of birth weight by ultrasound-estimated fetal weight: a comparison between single and repeated estimates

Objectives: Ultrasound estimation of fetal weight is used for diagnosing intrauterine growth retardation. The aim of the present study was to assess the accuracy of birth weight prediction by use of a single or repeated estimations of fetal weight in the third trimester. Study Design: 1000 pregnant women considered at risk were scheduled to ultrasound estimation of fetal weight, using Warsof's formula, at 28, 31, 34 and 37 weeks of gestation. The 421 pregnancies with term delivery and complete set of ultrasound examinations and 57 pregnancies with preterm delivery with ultrasound examination at 16 and 28 weeks and once more before delivery were included in the present analysis. Results: The accuracy of birth weight prediction improved significantly for every three weeks from 28 to 37 weeks of gestation in the term infants. Prediction based on the average of repeated weight estimates or linear extrapolation from two estimates or extrapolation by a second order polynomium fitted to four estimates did not improve accuracy compared to prediction based on the last estimate before delivery. Conclusion: When more than one ultrasound estimation of fetal weight are available, prediction of birth weight in relation to gestational age should be based on the last ultrasound examination only.