This study investigates the relationship between maternal pre-pregnancy body mass index (BMI) and 26 birth defects identified through the Florida Birth Defects Registry. Pre-pregnancy BMI (kg/m(2)) was categorised into underweight (<18.5), normal weight (18.5-24.9), overweight (25.0-29.9), and obese (≥30.0) among Florida resident mothers without pre-gestational diabetes who gave birth to singleton infants from March 2004 through December 2009. Obesity was classified as obese I (30.0-34.9), obese II (35.0-39.9), and obese III (≥40.0). Logistic regression was used to calculate the adjusted odds ratios and 95% confidence interval, representing the association between pre-pregnancy BMI and each of the 26 specific birth defects (and an 'any birth defect' composite). Models were adjusted for maternal age, race/ethnicity, education, smoking, marital status, and nativity. The livebirth prevalence of any birth defect increased with increasing BMI, from 3.9% among underweight women to 5.3% among obese III women (P < 0.001). Results show a direct dose-response relationship between maternal pre-pregnancy BMI and 10 defects under study (cleft palate without cleft lip, diaphragmatic hernia, hydrocephalus without spina bifida, hypoplastic left heart syndrome, pulmonary valve atresia and stenosis, pyloric stenosis, rectal and large intestinal atresia/stenosis, transposition of great arteries, tetralogy of Fallot, and ventricular septal defects) and the 'any birth defect' category. Conversely, gastroschisis exhibited a statistically significant inverse relationship with pre-pregnancy BMI. This study provides evidence of the increasing risk of birth defect-affected pregnancy with increasing pre-pregnancy obesity. Reducing pre-pregnancy obesity, even among obese women, may reduce the occurrence of birth defects. © 2013 John Wiley & Sons Ltd.
