To examine the differences and trends of outcomes of preterm boys and girls born at <29 weeks’ gestation.

A retrospective cohort study.

Data collected by the Canadian Neonatal Network.

Neonates born at <29 weeks’ gestation between January 2007 and December 2016.

We examined rate differences in mortality, major morbidities (bronchopulmonary dysplasia, severe brain injury, retinopathy of prematurity, necrotising enterocolitis and late-onset sepsis) and care practices (antenatal steroids, magnesium sulfate, maternal antibiotics, ventilation and surfactant administration) between boys and girls and evaluated trends in these rate differences over the study period. Our primary outcome was a composite of mortality and any one of the five morbidities.

Our study included 8219 boys and 6934 girls with median gestational age of 26 (IQR 25–28) weeks. The composite of death or major morbidity was more common in boys (adjusted risk ratio 1.07, 95% CI 1.05 to 1.10) and remained higher in boys over the study period. The gap between boys and girls for mortality, however, decreased over time: the slope for boys was −0.043 (95% CI −0.071 to −0.015) and for girls was −0.012 (95% CI −0.045 to 0.020) (p=0.04). All other morbidities remained higher in boys. Care practices changed at similar rates between the sexes.

The difference between the mortality rates for boys and girls decreased over the study period but the difference between rates of the major morbidities was unchanged. More research is needed to understand biological differences and outcome disparities.