We investigated the association between prepregnancy maternal body mass index 
(BMI) and preterm delivery (PTD). The study included 44,421 American women 
presenting for care in Saint Louis, Missouri between 1990 and 2006. Only 
singleton gestations were included. The authors examined the associations 
between categories of BMI with PTD <37 and <34 weeks, respectively. A stratified 
analysis by subtypes of PTD was also performed. The subtypes of PTD evaluated 
included spontaneous PTD without preterm premature rupture of membranes (PPROM), 
PPROM, and indicated PTD. Univariate and multivariable analyses were used to 
estimate the association between maternal BMI categories and PTD <37 weeks, PTD 
<34 weeks, and subtypes of PTD. Among women meeting the inclusion criteria, PTD 
<37 occurred in 4783 (10.8%) and PTD <34 weeks in 1132 (2.5%). Being underweight 
was associated with increased risks of PTD <37 weeks (adjusted odd ratio [OR] = 
1.3, 95% confidence interval [CI]: 1.2, 1.5). Being obese was associated with 
decreased risks of spontaneous PTD without PPROM <37 weeks (adjusted OR = 0.8, 
95% CI: 0.7, 0.9) and increased risk of PPROM <37 weeks (adjusted OR = 1.3, 95% 
CI: 1.1, 1.6) and PPROM <34 weeks (adjusted OR = 1.4, 95% CI: 1.0, 2.0). 
Prepregnancy obesity increases the risk of PPROM and decreases risk of 
spontaneous PTD without PPROM.