Sex Differences in Mortality and Morbidity of Infants Born at Less Than 30 Weeks' Gestation

To examine whether changes in mortality and morbidities have benefited male more than female infants.

Infants of gestational ages 22 to 29 weeks born between January 2006 and December 2016 at a Vermont Oxford Network center in the United States were studied. We examined mortality and morbidity rate differences and 95% confidence intervals by sex and birth year. We tested temporal differences in mortality and morbidity rates between boys and girls by means of a likelihood ratio test (LRT) on nested binomial regression models with log links.

A total of 205 750 infants were studied; 97 048 (47.2%) infants were girls. The rate for mortality and chronic lung disease decreased over time faster for boys than for girls (LRT P < .001 for mortality; P = .006 for lung disease). Restricting to centers that remained throughout the entire study period did not change all the above but additionally revealed a significant year-sex interaction for respiratory distress syndrome, with a faster decline among boys (LRT P = .04). Morbidities, including patent ductus arteriosus, necrotizing enterocolitis, early-onset sepsis, late-onset sepsis, severe intraventricular hemorrhage, severe retinopathy of prematurity, and pneumothorax, revealed a constant rate difference between boys and girls over time.

Compared with girls, male infants born at <30 weeks' gestation experienced faster declines in mortality, respiratory distress syndrome, and chronic lung disease over an 11-year period. Future research should investigate which causes of death declined among boys and whether their improved survival has been accompanied by a change in their neurodevelopmental impairment rate.