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