Discussion
The association of pre-pregnancy BMI with adverse pregnancy outcomes is not fully understand, especially in low-income and middle-income countries, where levels of maternal overweight/obesity is increasing and underweight is still high.14 In this large cohort study conducted in China, we found that compared with women with normal weight, pre-pregnancy underweight was positively associated with risk of PTB and SGA but inversely associated with risk of LGA, primary caesarean delivery and stillbirth; overweight was positively associated with risk of LGA, primary caesarean delivery and stillbirth but inversely associated with risk of SGA and shoulder dystocia or birth injury; and obesity was positively associated with risk of PTB, LGA and primary caesarean delivery but inversely associated with risk of SGA. However, these associations differ according to maternal age.