Does Fetal Gender Affect Risk of Preterm Birth?

February 11, 2015

SMFM 2015: New research shows that fetuses of a certain sex are at increased risk for spontaneous preterm birth.

The underlying causes of spontaneous preterm birth are still largely unknown. There are various risk factors for spontaneous preterm birth, but the primary risk factor is a previous preterm delivery. The sex of the unborn baby also seems to play a role in the process of being born prematurely, according to research presented last week in San Diego at The Pregnancy Meeting, the Society for Maternal-Fetal Medicine’s annual meeting.

The research was based on nearly 2 million (1,947,266) singleton births by white women between 1999 and 2010 involving a live fetus at the onset of labor. Cases with congenital anomalies or unknown fetal sex were excluded. The relative risk ratios for gender per week of gestation were assessed, as was gender-related risk on adverse neonatal outcomes. These outcomes were perinatal mortality and a composite of neonatal morbidity, including neonatal intensive care admission, sepsis, meconium aspiration, necrotizing enterocolitis, respiratory distress syndrome, and intraventricular hemorrhage.

Onset of labor was categorized as spontaneous with intact membranes, prelabor rupture of membranes (PROM), and induction of labor or elective c-section.

When looking at spontaneous preterm birth with intact membranes, male fetuses were 50% more likely than female fetuses to be born prematurely, with a peak between 28 and 31 weeks' gestation (RR, 1.5; 95% CI, 1.3-1.7). In addition, male fetuses were at increased risk for PPROM between 26 and 37 weeks' gestation (RR 1.2; 95 CI, 1.16-1.23). The sex of the fetus had no effect on medically indicated preterm birth.

Neonatal mortality was similar between male and female fetuses with comparable gestational ages at birth. However, male fetuses were more likely than female fetuses to have a composite neonatal morbidity when born on or after 29 weeks' gestation, with a peak at 37 to 38 weeks' gestation (RR, 1.4; 95% CI, 1.2-1.5). Plainly speaking, if a boy and girl are born with comparable gestational ages, the boy is more at risk for becoming seriously ill than the girl.

“This study provides important insight into the differences in risk factors based on gender,” explained Myrthe Peelen, MD, one of the researchers of the study and who is affiliated 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.

Peelen M, Kazemier B, Ravelli A, et al. Impact of fetal gender on the risk of preterm birth. Abstract no 80. Presented at: The Pregnancy Meeting, the Society for Maternal-Fetal Medicine’s annual meeting; February 7, 2015; San Diego, Calif.