Impact of fetal gender on the risk of preterm birth, a national cohort study

Introduction. Fetal gender is associated with preterm birth; however, a proper subdivision by onset of labor and corresponding neonatal outcome by week of gestation is lacking. Material and methods. Data from the Netherlands Perinatal Registry (1999-2010) were used to calculate relative risk ratios for gender by week of gestation and gender-related risk on adverse neonatal outcomes using a moving average technique. White European women with an alive fetus at onset of labor were included. Adverse neonatal outcomes were defined as neonatal mortality and a composite of neonatal morbidity. Onset of labor was categorized as spontaneous onset with intact membranes, premature rupture of membranes, and induction or elective cesarean section. Results. The study population comprised 1 736 615 singleton deliveries (25(+0)-42(+6) weeks). Male fetuses were at increased risk of spontaneous preterm birth with intact membranes compared with a female fetus with a peak between 27 and 31 weeks [relative risk (RR) 1.5; 95% CI 1.4-1.6]. Male fetuses were also at increased risk of preterm premature rupture of membranes between 27 and 37 weeks (RR 1.2; 95% CI 1.16-1.23). No gender effect was seen for medically indicated preterm birth. No significant differences were seen for neonatal mortality. Males were at significantly increased risk of composite neonatal morbidity from 29 weeks onwards (RR 1.3; 95% CI 1.3-1.4). Conclusions. Male fetal gender is a relevant risk factor for spontaneous preterm birth, both for intact membranes and for preterm premature rupture of membranes in white European women. In addition, male infants are at increased risk of neonatal morbidity.