Sensitivity and subgroup analysis
To examine the robustness of the association of pre-pregnancy BMI with
adverse pregnancy outcomes, we preformed other two sensitivity analysis
with additional adjustment for the length of time from pre-pregnancy
examination to the last menstrual period (continuous data) or inclusion
of women with self-reported perceived economic pressure (yes or no).
In the subgroup analysis, we divided women into different subgroups on
the basis of maternal age. Among these age subgroups, we examined the
associations of pre-pregnancy BMI with adverse pregnancy outcomes except
for stillbirth. Among all the sensitivity and subgroup analysis, we
adjusted for the all covariates listed in model 4.
Data were missing in first pregnancy (2910, 0.4%), primipara (2910,
0.4%), active smoke before pregnancy (4960, 0.7%), passive smoke
(4939, 0.7%), husband smoke before pregnancy (23837, 3.6%), alcohol
before pregnancy (6777, 1.07%), active smoke during early-stage
pregnancy (22225, 3.3%), husband smoke during early-stage pregnancy
(22742, 3.4%), alcohol during early-stage pregnancy (22775, 3.4%). We
imputed these missing covariates by using the multiple imputation
methodology based on other socio-demographic covariates. The
significance level was set at 0.05 and all tests were two-sided.
Statistical analyses were conducted by using Stata (Version 14.0) and
Statistic software R (version 3.5.2).