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. © 2004 Elsevier Inc. All rights reserved.
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