A rough start especially for boys: the influence of fetal sex on delivery mode.

In Austria, more than a third of all births are performed with the assistance of vacuum extraction or a caesarean section. Consistent with the male disadvantage hypothesis, many indications for obstetric intervention, including cephalopelvic disproportion or placental abruption, affect male fetuses more frequently than female ones. Thus, we hypothesized that male fetuses show a higher rate of obstetric intervention than female fetuses. This retrospective, single-center study was based on 11749 singleton live births taking place in Vienna, Austria between 2010 and 2020. Maternal (age, body height, weight, parity, nicotine use), obstetric (fetal presentation, gestational length, delivery mode), and neonatal parameters (sex, birthweight, birth length, head circumference) were included in the analysis. Both instrumental vaginal delivery (6.8% vs. 4.8%) and emergency cesarean section (9.4% vs. 8.2%) occurred significantly more often in males than females (p < 0.001). This association between fetal sex and delivery mode persisted despite controlling for confounding factors such as head circumference, with males having a 1.51 resp. 1.22 increased risk of instrumental vaginal delivery (p < 0.001) resp. emergency cesarean section (p = 0.006) compared to females. Male sex is a major risk factor of birth complications and consequently higher rates of instrumental vaginal delivery and emergency cesarean section.