Association between fetal sex, birthweight percentile and adverse pregnancy outcome

Introduction: The objective was to evaluate the association between fetal sex and adverse pregnancy outcome, while correcting for fetal growth and gestational age at delivery.

Material and methods: Data from the Netherlands Perinatal Registry (1999-2010) were used. The study population comprised all white European women with a singleton delivery between 25⁺⁰ and 42⁺⁶ weeks of gestation. Fetuses with structural or chromosomal abnormalities were excluded. Outcomes were antepartum death, intrapartum/neonatal death (from onset of labor until 28 days after birth), perinatal death (antepartum death or intrapartum/neonatal death), a composite of neonatal morbidity (including infant respiratory distress syndrome, sepsis, necrotizing enterocolitis, meconium aspiration, persistent pulmonary hypertension of the newborn, periventricular leukomalacia, Apgar score <7 at 5 minutes, and intracranial hemorrhage) and a composite adverse neonatal outcome (perinatal death or neonatal morbidity). Outcomes were expressed stratified by birthweight percentile (<p10 [small for gestation], p10-90 [normal weight], >p90 [large for gestation]) and gestational age at delivery (25⁺⁰ -27⁺⁶ , 28⁺⁰ -31⁺⁶ , 32⁺⁰ -36⁺⁶ , 37⁺⁰ -42⁺⁶ weeks). The association between fetal sex and outcome was assessed using the fetus at risk approach.

Results: We studied 1 742 831 pregnant women. We found no increased risk of antepartum, intrapartum/neonatal and perinatal death in normal weight and large-for-gestation males born after 28⁺⁰ weeks compared with females. We found an increased risk of antepartum death among small-for-gestation males born after 28⁺⁰ weeks (relative risk [RR] 1.16-1.40). All males born after 32⁺⁰ weeks of gestation suffered more neonatal morbidity than females regardless of birthweight percentile (RR 1.07-1.34). Infant respiratory distress syndrome, sepsis, persistent pulmonary hypertension of the newborn, Apgar score <7 at 5 minutes, and intracranial hemorrhage all occurred more often in males than in females.

Conclusions: Small-for-gestation males have an increased risk of antepartum death and all males born after 32⁺⁰ weeks of gestation have an increased risk of neonatal morbidity compared with females. In contrast to findings in previous studies we found no increased risk of antepartum, intrapartum/neonatal or perinatal death in normal weight and large-for-gestation males born after 28+0 weeks.