Objective: This study was undertaken to explore the association between fetal sex, mode of onset of labor, and principal cause of very preterm birth. Study design The analysis uses data on 2624 very preterm singleton births less than 33 weeks' gestation from a prospective study of all very preterm births in 9 French regions in 1997.
Results: Fifty-seven percent of the spontaneous births were male versus 50.8% of births after medical decision (P=.005). This pattern was explained by sex differences in causes of preterm birth. Male infants had a greater incidence of very preterm birth after spontaneous labor (relative risk [RR]=1.42 [1.21-1.66]), but one third less risk of indicated preterm birth associated with hypertension both with and without growth restriction (RR=0.73 [0.55-0.97] and 0.77 [0.60-0.97]).
Conclusion: These results support previous reports of greater male susceptibility to preterm labor. The finding that pregnancies carrying female infants have a greater predisposition to indicated very preterm birth associated with hypertension has not been reported previously and merits further study.