Impact of fetal gender on risk of preterm birth

Preterm birth, a delivery before 37 weeks of gestation, is one of the most serious obstetric complications affecting around 15 million pregnancies worldwide with more than one million newborn deaths each year due to complications of prematurity. So far, the underlying causes for preterm birth are still largely unknown.

Preterm birth can either occur spontaneously or can be induced for medical reasons. There are various risk factors for preterm birth, of which a previous preterm birth is one of the most important. The gender of the unborn baby also seems to play a role in the process of being born prematurely.

The study, titled Impact of Fetal Gender on the Risk of Preterm, a Dutch nationwide study, looked at 1,947,266 singleton births by Caucasian women between 1999 and 2010 with a fetus alive at the onset of labor. Cases with congenital anomalies or unknown fetal gender were excluded. The relative risk ratios for gender per week of gestation were assessed as well as gender related risk on adverse neonatal outcomes. These outcomes were perinatal mortality and a composite of neonatal morbidity (defined as neonatal intensive care admission, sepsis, meconium aspiration, necrotizing enterocolitis, respiratory distress syndrome or intraventricular hemorrhage).

The study concluded that male fetuses were at increased risk of spontaneous preterm birth as well as preterm premature rupture of membranes. There were no significant differences between male and female fetuses born at comparable gestational ages regarding neonatal mortality; however, males were at significantly increased risk of composite neonatal morbidity compared to females from 29 weeks onward with a peak at 37-38 weeks. So, if a boy and girl are born with comparable gestational ages, the boy is more at risk of becoming seriously ill than the girl.

"This study provides important insight into the differences in risk factors based on gender," explained Myrthe Peelen, M.D. who is one of the researchers of the study and is also with the Academic Medical Center in Amsterdam. "Patients with a history of preterm birth should be monitored, particularly if they're having boys now after a preterm birth of a girl," added Peelen.