Demographic and obstetric characteristics of the study cohort by fetal sex.

aMean+/−SD;bincome csmoking defined as one or more cigarettes per day;dincludes both elective and in-labor sections;eall deliveries excluding elective Cesarean-section deliveries;fstage 1 and 2 labor combined;gthe number of pregnancies included for each category or variable are indicated in parentheses;hNS,not significant (P>0.05 by Mann-Whitney U-test for group median comparisons or Fisher's Exact test for categorical variables).