Fetal Gender of the First Born and the Recurrent Risk of Spontaneous Preterm Birth

Objective To study, in women with a spontaneous preterm birth (sPTB) in the first pregnancy, the effect of fetal sex in that first pregnancy on the recurrent sPTB risk. Study Design A nationwide retrospective cohort study (data from National Perinatal Registry) on all women with two sequential singleton pregnancies (1999-2009) with the first delivery ending in sPTB <37 weeks. We used logistic regression analysis to study the association between fetal gender in the first pregnancy and the risk of recurrent sPTB. We repeated the analysis for sPTB <32 weeks. Results The overall incidence of sPTB <37 weeks in the first pregnancy was 4.5% (15,351/343,853). Among those 15,351 women, the risk of recurrent sPTB <37 weeks was increased when the first fetus was female compared when that fetus was male (15.8 vs. 15.2%; adjusted odds ratio [aOR] 1.2; 95% confidence interval [CI] 1.05-1.3). A similar effect was seen for sPTB <32weeks (8.2 vs. 5.9%; aOR 4.5; 95% CI 1.5-13). Conclusion Women who suffer sPTB of a female fetus have an increased risk of recurrent sPTB compared with women who suffer sPTB of a male fetus. This information provides proof for the hypothesis that sPTB is due to an independent maternal and fetal factor.