The purpose of this study was to evaluate the association between fetal gender and prolonged pregnancy.All deliveries in Sweden between 1987 and 1996 were evaluated for participation in this study. Inclusion criteria included (1) singleton pregnancy, (2) the absence of apparent congenital or chromosomal anomalies, (3) accurate dating established by early second trimester ultrasound examination, and (4) gestational age at delivery of > or =37 weeks (ie, > or =259 days). Initially, we calculated the mean gestational age at delivery and the percentage of prolonged pregnancies by fetal gender. Subsequently, the Mantel-Haenszel chi-square analysis was used to calculate the weekly odds ratios and their corresponding 95% confidence intervals for the delivery of a male fetus beyond 37 weeks of gestation.The study population comprised 656,423 deliveries; 333,192 were male deliveries, and 323,231 were female deliveries (male/female ratio, 1.03). The mean gestational age at delivery was significantly higher in male fetuses (280.6 +/- 8.9 days vs 279.8 +/- 8.6 days, respectively; P or =41 weeks of gestation and 7.6% vs 5.5% [P or =42 weeks of gestation, respectively). The weekly odds ratios for a delivery of a male fetus beyond term were 1.14, 1.39, and 1.50 at 41, 42, and 43 weeks, respectively.Male gender significantly predisposes to the prolongation of pregnancy to the extent that, by 43 weeks of gestation, there are 3 male deliveries for every 2 female deliveries.