Examination of the Association Between Male Gender and Preterm Delivery

There is a small but consistent excess of preterm births of male fetuses. Compared to female fetuses, male fetuses are also at increased risk of congenital malformations, miscarriage, delivery by cesarean section, need for resuscitation and intensive care at birth, death at birth, and development of cerebral palsy early in life. Several possible risk factors for male preterm delivery have been proposed. This retrospective study tested potential risk factors for male preterm births by reviewing a database of all deliveries >23 weeks' gestation occurring over an 11-year period at a teaching hospital. Demographic, antenatal, intrapartum, and outcome variables for preterm male and female neonates born at 3 different gestational age ranges were compared: extremely preterm (24-28 weeks), severely preterm (29-32 weeks), and moderately preterm (33-36 weeks). Exclusions included pregnancies with major congenital abnormalities, terminations, multiple pregnancies and those with fetuses of indeterminate gender. Odds ratios (OR) with 95% confidence intervals (CIs) for preterm delivery at each of the 3 gestational ages were calculated, using the full-term cohort (37-42 weeks) for comparison. Adjusted OR (aORs) with 95% CIs were calculated using binary logistic regression to test potentially causal factors associated with male sex that might account for the sex difference. Of the 75,725 deliveries examined, 4003 (5.3%) were preterm. Preterm birth occurred more frequently among males in all 3 gestation age ranges (OR, 1.13; 95% CI, 1.06-1.20). The increase in preterm births was due to spontaneous (OR, 1.30; 95% CI, 1.19-1.42) but not iatrogenic (OR, 0.96; 95% CI, 0.87-1.05) preterm deliveries. Preeclampsia was more likely to occur among preterm females (P < 0.001), and male fetuses were 130 grams larger. The increased risk of spontaneous preterm labor among pregnancies with male fetuses remained significant after adjustment for the larger male birth weight and the greater incidence of nulliparity and higher frequency of urinary tract infection and placental abruption among the mothers of male fetuses. These data indicate that the increased risk of preterm birth among males is due to an increased incidence of spontaneous preterm labor. No causal factors have been identified.