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.