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.