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).