We compared
maternal and neonatal characteristics among
the three periods using the Mantel–Haenszel
chi-square test for categorical variables and the
Kruskal–Wallis test for continuous variables. We
performed post hoc subgroup analyses to evaluate
potential differences in overall and immaturity-
related mortality due to the addition and
attrition of centers over time, and we performed
sensitivity analyses to evaluate the effects of
potential misclassification of deaths due to immaturity,
the respiratory distress syndrome, or
bronchopulmonary dysplasia (see the Methods section
in the Supplementary Appendix). Additional
analyses examined causes of death that were
complicated by either CNS injury or infection for
which another primary cause was listed.