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

aMean+/−SD; bincome <$24,000 per annum; 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).