Participant characteristics
By 31st December 2017, 727999 women had pregnancy outcomes. We excluded 14096 women who did not measure pre-pregnancy BMI; 41943 women with chronic diseases; 1995 women with multiple births and 864 women without any data on gestational week, birthweight or delivery methods. The remaining 669101 participants from 121 counties in 21 cities were included in the final analysis to examine the association of pre-pregnancy BMI with PTB, LGA birth and SGA birth. After additional exclusion of 145 women without data on delivery method, 668956 participants were included in the analysis to examine the association of pre-pregnancy BMI with primary caesarean delivery, should dystocia or birth or birth injury. A subgroup of 256882 abstracted from 46 counties 13 cities, who had data on stillbirth was included in the analysis to investigate the association of pre-pregnancy BMI with stillbirth. Figure S1 shows the selection of participants for the present study. The sample size and the proportion of the migrant population in each city are shown in Table S1-S3.
Characteristics of the women who had information on gestational age and infants’ birthweight are summarized in Table S4. Among the 669101 women included, 136287 (20.3%) were underweight, 69819 (10.4%) overweight and 14556 (2.2%) obesity. The distribution of BMI categories with respect to different baseline characteristics were all significant different (P <0.05). Characteristics of 668956 women who had information on delivery methods and 256882 women who had information on stillbirth are shown in Supplemental S5-S6.