Association between maternal prepregnancy body mass index and risk of preterm birth in more than 1 million Asian American mothers

Background Asian Americans are among the fastest growing subpopulations in the United States. However, evidence about maternal prepregnancy body mass index (BMI) and preterm birth among Asian Americans is lacking. Methods This population-based study used nationwide birth certificate data from the US National Vital Statistics System 2014 to 2018. All Asian American mothers who had a singleton live birth were included. According to Asian-specific cutoffs, maternal prepregnancy BMI was classified into underweight (BMI < 18.5 kg/m(2)), normal weight (BMI 18.5-22.9 kg/m(2)), overweight (BMI 23.0-27.4 kg/m(2)), class I obesity (BMI 27.5-32.4 kg/m(2)), class II obesity (BMI 32.5-37.4 kg/m(2)), and class III obesity (BMI >= 37.5 kg/m(2)). Preterm birth was defined as gestational age less than 37 weeks. Multivariable logistic regression models were used to estimate the odds ratio (OR) of preterm birth. Results We included 1 081 341 Asian American mother-infant pairs. The rate of preterm birth was 6.51% (n = 70 434). The rate of maternal prepregnancy overweight and obesity was 46.80% (n = 506 042). Compared with mothers with normal weight, the adjusted OR of preterm delivery was 1.04 (95% CI, 1.01-1.07) for underweight mothers, 1.18 (95% CI, 1.16-1.20) for overweight mothers, 1.41 (95% CI, 1.37-1.44) for mothers with class I obesity, 1.69 (95% CI, 1.63-1.76) for mothers with class II obesity, and 1.78 (95% CI, 1.66-1.90) for mothers with class III obesity. Similar patterns of associations were observed in Asian American mothers across different country origins. Conclusions Among Asian American mothers, maternal prepregnancy overweight or obesity, defined by Asian-specific, lower BMI cutoffs, was significantly associated with an increased risk of preterm birth. The risk of preterm birth increased with increasing obesity severity. These findings highlight the importance of using Asian-specific BMI cutoffs in assessing risk of preterm birth among Asian American mothers.