Investigating the association between prepregnancy body mass index and adverse pregnancy outcomes: a large cohort study of 536 098 Chinese pregnant women in rural China

Abstract

Objective: Unhealthy maternal weight before pregnancy increases the risk of various adverse pregnancy outcomes. We conducted a nutrition survey to provide baseline data on the prepregnant nutritional status of mothers in order to better understand the association between prepregnancy maternal body mass index (BMI) and adverse pregnancy outcomes.

Design: A large, prospective, population-based cohort study.

Setting: Data from the National Free Preconception Health Examination Project (NFPHEP) in China during 2010-2012.

Participants: 536 098 pregnant women out of 2 120 131 were evaluated.

Primary and secondary outcome measures: The primary adverse pregnancy outcomes included preterm birth (PTB), low birth weight (LBW), spontaneous miscarriage (SM), ectopic pregnancy (EP) and stillbirth (SB). A χ(2) test was used to compare the prevalence of each BMI category during 2010-2012. Univariable and multiple logistic regression analyses were performed to assess the association between prepregnancy BMI and various adverse pregnancy outcomes.

Results: Between 2010 and 2012, the average BMI decreased from 21.31 to 21.16, while underweight prevalence increased from 10.40% to 14.14%. An age-stratified subgroup analysis indicated that the underweight prevalence increased from 13.52% to 17.02% among women aged 21-24 and from 10.72% to 13.71% among women aged 25-34. Overweight prevalence increased from 9.84% to 10.75% (25-34 years) and from 17.10% to 19.20% (35-49 years). Obesity prevalence increased from 2.17% to 2.42% and from 4% to 4.2% among women aged 25-34 and 35-49 respectively. Prepregnancy underweight was associated with PTB, LBW and SM; overweight women had an increased risk of LBW; obese women had a higher risk of LBW, SM, EP and SB.

Conclusions: While the average prepregnancy BMI decreased, the prevalence of underweight individuals in a very large population significantly increased. The abnormal prepregnancy BMIs were associated with increased risks of adverse pregnancy outcomes. Most notably, underweight prepregnant women appeared to be at a greater risk of developing adverse pregnancy outcomes in China's rural areas.

Keywords: adverse pregnancy outcomes; body mass index; cohort study; pre-pregnancy.