AbstractObjectivesTo explore the relationship between cerebroplacental Doppler impedance index and birth weight in postdates pregnancies, and to evaluate the use of a combination of Doppler parameters and ultrasound biometry in the prediction of large‐for‐gestational age (LGA) fetuses at 41 weeks of gestation.MethodsThe pulsatility indices of the umbilical (UA‐PI) and middle cerebral (MCA‐PI) arteries, the cerebroplacental pulsatility index ratio (CPR) and the estimated fetal weight (EFW) were obtained in a cohort of 181 ultrasound‐dated pregnancies at 41 weeks' gestation, 2 days before induction of delivery. A regression equation was established and the correlation between umbilical artery impedance and different birth‐weight centile groups was determined. A receiver–operating characteristics (ROC) curve was used to compare prediction of LGA fetuses using biometry alone with that using biometry and UA‐PI.ResultsUA‐PI was inversely related to EFW (Spearson's correlation coefficient rho = −0.28, P < 0.001). Logistic regression showed an independent contribution of UA‐PI to the birth‐weight estimation (birth weight = 1356.8 − 232.0 × UA‐PI + 0.65 × EFW). On ROC curve analysis, the prediction of LGA with the regression equation was comparable to that using ultrasound biometry alone.ConclusionUA‐PI was inversely correlated to EFW, but the combination of ultrasound biometry and UA‐PI compared with biometry alone showed similar prediction of LGA fetuses in postdates pregnancies. Further prospective trials on larger populations or groups with a higher prevalence of LGA fetuses would be needed to validate the use of the new formula. Copyright © 2005 ISUOG. Published by John Wiley & Sons, Ltd.