Clubfoot is a common structural malformation, occurring in approximately 1/1,000 live births. Previous studies of sociodemographic and pregnancy-related risk factors have been inconsistent, with the exception of the strong male preponderance and association with primiparity. Hypotheses for clubfoot pathogenesis include fetal constraint, Mendelian-inheritance, and vascular disruption, but its etiology remains elusive. We conducted a population-based case-control study of clubfoot in North Carolina, Massachusetts, and New York from 2007 to 2011. Mothers of 677 clubfoot cases and 2,037 non-malformed controls were interviewed within 1 year of delivery about socio-demographic and reproductive factors. Cases and controls were compared for child's sex, maternal age, education, cohabitation status, race/ethnicity, state, gravidity, parity, body mass index (BMI), and these pregnancy-related conditions: oligohydramnios, breech delivery, bicornuate uterus, plural birth, early amniocentesis (<16 weeks), chorionic villous sampling (CVS), and plural gestation with fetal loss. Odds ratios (ORs) and 95% confidence intervals (CIs) were adjusted for state. Cases were more likely to be male (OR: 2.7; 2.2-3.3) and born to primiparous mothers (1.4; 1.2-1.7) and mothers with BMI ≥30 kg/m(2) (1.4; 1.1-1.8). These associations were greatest in isolated and bilateral cases. ORs for the pregnancy-related conditions ranged from 1.3 (breech delivery) to 5.6 (early amniocentesis). Positive associations with high BMI were confined to cases with a marker of fetal constraint (oligohydramnios, breech delivery, bicornuate uterus, plural birth), inheritance (family history in 1st degree relative), or vascular disruption (early amniocentesis, CVS, plural gestation with fetal loss). Pathogenetic factors associated with obesity may be in the causal pathway for clubfoot. © 2013 Wiley Periodicals, Inc.