Preterm birth (PTB) and food insecurity are two of the most significant public health crises in the United States. Effects of being underweight among populations with low food security are not well understood. We assess whether the protective effect of gestational weight gain (GWG) for women with low prepregnancy body mass index (BMI) differs by accessibility to sources of healthy food.
 Population-based retrospective cohort study using Ohio birth records analyzing all live births, 2006 to 2015. Analyses were stratified by maternal BMI (underweight, normal, overweight, and obese), Institute of Medicine (IOM) recommended GWG (under vs. met), and whether the U.S. Department of Agriculture (USDA) classified the residential census tract for each birth as a food desert. Food access data were retrieved from the USDA’s 2018 Food Access Research Atlas. Covariates were selected using least absolute shrinkage and selection operator regression. Logistic regression models estimated the risk ratio (RR) of PTB for each group based on under or exceeded recommended GWG (reference = met), adjusting for coexisting risk factors.
 Analysis was performed on 1,124,299 births. PTB risk was highest for underweight women below GWG recommendations (no food desert: 21.3%, RR = 2.15, 95% confidence interval [CI]: 1.81-2.57; food desert: 21.0%, RR = 1.46, 95% CI: 0.96-2.21). Underweight women living in food deserts who exceeded GWG recommendations had lower PTB risk than those who met GWG recommendations (13.5 vs. 14.3%, RR = 0.85, 95% CI: 0.51-1.41). Factors other than GWG significantly associated with PTB included in the adjusted analysis include maternal age and race, education, marital status, interpregnancy interval, and presence of prepregnancy diabetes or hypertension.
 Underweight women who do not meet GWG recommendations are at high risk for PTB. Increasing pregnancy weight gain to a level that exceeds IOM recommendations was not associated with a reduction in PTB risk for underweight women who reside in food deserts compared with women who met GWG recommendations.

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