Implications
Our findings have important clinical and public health implications. Abnormal pre-pregnancy BMI is common among reproductive age women around the world. Evidence of management of women with different weight in pregnancy were mainly from west countries where have high prevalence of overweight and obesity (including severe obesity) and have different BMI classification from China,6 and which may not adopt to other developing countries like China. Our findings from more than 660000 women confirmed that compared with women with normal weight, a statistically significant increase in risk estimate by 6% of PTB, 23% of SGA in underweight women; 17% of LGA, 18% of primary caesarean delivery, and 44% of stillbirth in overweight women; and 12% of PTB, 32% of LGA, and 45% of primary caesarean delivery in obese women. This suggested that clinical evidence-based recommendation and counselling for management of BMI before and during pregnancy among reproductive age women might be necessary for reducing the risk of adverse pregnancy outcomes in China. The finding also suggested the recommendation and counselling should tailor to maternal age, as the association of pre-pregnancy BMI with adverse pregnancy differed according to maternal age.