Fetal sex-related preterm birth and placental cell turnover in dichorionic twins.

In dichorionic twins, male-male pairs exhibited the highest maternal peripheral blood cell-free fetal DNA and seemingly the greatest risk of preterm birth and premature rupture of membranes. The preterm birth rate in other twin pregnancies was significantly lower than that in male-male pairs. Within opposite sex twins, male fetal placentas exhibited higher apoptosis, while female placentas showed a propensity for greater proliferation. Thus, the protective effect might come from the lower placental apoptosis index observed in female fetuses, although the exact molecular pathways underlying this sex specific modulation remained to be elucidated and represented an important direction for future research.