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

a<p>Mean+/−SD;</p>b<p>income <$24,000 per annum;</p>c<p>smoking defined as one or more cigarettes per day;</p>d<p>includes both elective and in-labor sections;</p>e<p>all deliveries excluding elective Cesarean-section deliveries;</p>f<p>stage 1 and 2 labor combined;</p>g<p>the number of pregnancies included for each category or variable are indicated in parentheses;</p>h<p>NS,</p><p>not significant (<i>P</i>>0.05 by Mann-Whitney U-test for group median comparisons or Fisher's Exact test for categorical variables).