Statistical analysis
Mean and standard deviation were reported for continuous variables, and
frequencies and percentage were reported for categorical variables.
Chi-square tests were employed to compare the distribution of BMI
categories according to different baseline characteristics. Log-binomial
models based on Generalized Estimating Equations (GEE) were employed to
estimate the adjusted risk ratios (RRs) and 95% CIs of the six outcomes
for underweight, overweight and obesity. In each outcome, three models
were fitted.
In Model 1, we adjusted for participants’ sociodemographic
characteristics, including age at baseline (19-24 years, 25-29 years,
30-34 years, 35-39 years, or 40-50 years), ethnicity (Han or others),
educational level (primary school or below, junior high school, senior
high school or college or above), occupation (farmer, worker, servicer
or others), region (pearl river
delta, non-pearl river delta), migrant population (yes or no). In model
2, we additionally adjusted for history of pregnancy and adverse
pregnancy outcomes except for primary caesarean
delivery,13 including first pregnancy (yes or no),
primipara (yes or no); history of PTB (yes or no), miscarriage (yes or
no), induced abortion (yes or no), birth defects (yes or no), or
stillbirth (yes or no). In model 3, we additionally adjusted for the
lifestyles of the women and the husband, including smoking status of
husband before pregnancy and during the early stage of pregnancy (yes or
no), smoking and alcohol consumption of women before pregnancy and
during the early stage of pregnancy (yes or no), and passive smoking of
women before pregnancy (yes or no). Because infant’s sex is associated
with the six outcomes, we adjusted for this variable in all analysis in
addition to others listed.