Weight measurements
Gestational age was estimated based on the date of last menstruation period and confirmed by first trimester ultrasound date. Pre-pregnancy weight (kg) was based on self-reporting, while weight at every prenatal visit and at delivery was routinely measured to the nearest 0.1 kg using the available electronic weighing device in the prenatal care clinics. Height (cm) at the first prenatal visit was routinely measured to the nearest 0.1 cm using the available electronic stadiometer in the hospital. Pre-pregnancy body mass index (BMI; kg/m2) was calculated as pre-pregnancy weight (kg) divided by height (m)2 and categorized as underweight (<18.5 kg/m2), normal weight (18.5 to 24.9 kg/m2), overweight (25.0 to 29.9 kg/m2), and obese (≥ 30.0 kg/m2)[16]. However, due to the sporadic number of obese women, we analyze them together with overweight women in this study.
We defined the following 3 gestational intervals: ≤14, 24 to 28, >28 weeks. If a woman had more than 1 antenatal visit within an interval, we took her last weight measurements for that interval. GWG in early pregnancy was calculated as the antenatal weight up to ≤14 weeks minus the pre-pregnancy weight; GWG in mid pregnancy was calculated as the weight measured in 24 to 28 weeks’ intervals minus the last weight measured ≤14 week, and late pregnancy as last measurement of weight prior to delivery minus the weight measured in 24 to 28 weeks’ intervals. Total GWG was calculated as last measurement of weight before delivery minus pre-pregnancy weight. All GWG values were standardized into z-scores by gestational age, stratified by BMI categories. The means and standard deviations (SD) of GWGs in early, mid, late and whole pregnancy were used to convert the GWG values into z-scores. All GWG z-scores were first examined as continuous variables, and then categorized as < -1.0 (below), -1.0 to +1.0 (average) and > +1.0 (above) in data analyses.