Interpregnancy BMI change and interval
Maternal weight in kilograms and height in centimetres were routinely measured at the first antenatal appointment of the first and second deliveries. BMI, calculated as weight (in kilograms) divided by height (in metres) squared, at the first antenatal visit during the first and second pregnancies, was used to determine the interpregnancy BMI change. Given that gestational age at the first antenatal visit varied, the BMI measures were standardised separately in the first and second deliveries, by using linear regression with BMI at the first antenatal visit as dependent variable and gestational age centred at 12 weeks (i.e. the mean gestational age at the first antenatal visits) as the independent variable, calculating the residuals, and adding the residual values to the regression predicted mean BMI at 12 weeks. The difference between gestational-age-adjusted BMI at both visits was then calculated as the change in BMI from the first to second deliveries and further categorized as BMI stable -1 to <1 kg/m2, BMI loss >1 kg/m2, moderate BMI gain 1 to <3 kg/m2 and excess BMI gain ≥3 kg/m2. The gestational-age-adjusted BMI at 12 weeks was used to represent the pre-pregnancy BMI in both pregnancies and was categorized using cut-offs for Asian populations: underweight (<18.5 kg/m2), normal weight (18.5-22.9 kg/m2), overweight (23-27.49 kg/m2) and obese (≥27.5 kg/m2).11 The interpregnancy interval was calculated based on the period between the first delivery date and the second delivery conception date, which was derived by subtracting gestational age at delivery for the second birth from the duration between delivery dates of two consecutive births.12