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

ObjectiveTo 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 DesignA 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. ResultsThe 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). ConclusionWomen 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.