Elevated body mass index and decreased diet quality among women and risk of birth defects in their offspring.

BACKGROUND We examined whether risks of 32 birth defects were higher than expected in the presence of overweight or obese body mass index (BMI) and low diet quality, based on estimating individual and joint effects of these factors and calculating relative excess risk due to interaction. METHODS Analyses included mothers of 20,250 cases with birth defects and 8617 population-based controls without birth defects born from 1997 to 2009 and interviewed for the National Birth Defects Prevention Study. We used logistic regression to generate adjusted odds ratios (AORs) reflecting the combined effects of BMI and diet quality. We focused analyses on 16 birth defects (n = 11,868 cases, 8617 controls) for which initial results suggested an association with BMI or diet quality. RESULTS Relative to the reference group (normal weight women with not low diet quality, i.e., >lowest quartile), AORs for low diet quality among normal weight women tended to be >1, and AORs for overweight and obese women tended to be stronger among women who had low diet quality than not low diet quality. For 9/16 birth defects, AORs for obese women who had low diet quality-the group we hypothesized to have highest risk-were higher than other stratum-specific AORs. Most relative excess risk due to interactions were positive but small (<0.5), with confidence intervals that included zero. CONCLUSION These findings provide evidence for the hypothesis of highest birth defect risks among offspring to women who are obese and have low diet quality but insufficient evidence for an interaction of these factors in their contribution to risk.