Women’s characteristics
This study initially enrolled 7095 women with singleton first and second pregnancies. Of these, we excluded 831 women without BMI measured in one of the pregnancies (n=772) or in both pregnancies (n=59), leaving 6264 women in the final sample. Compared to excluded women, the included women tend to be older by only 0.5 years on average (28.4 vs 27.9 years, p=0.015). (Table S1) . All other background variables were similar between the women included and excluded from the analysis (each p>0.05)
The characteristics of participants are displayed in Table 1 . Of the 6264 women included, most were of Chinese ethnicity (41.5%), followed by Malay (30.4%) and Indian (10.6%), and other ethnicities that included mixed Asians or Caucasians. These women had a mean age of 28.4 years. They had a mean BMI of 23.8 kg/m2 at 12 weeks during the first pregnancy, which increased to 24.9 kg/m2 at 12 weeks during the second pregnancy. The interpregnancy BMI change had a median gain of 0.97 kg/m2, while the median interval between the first and second pregnancies was 1.44 years. Overall, 40.7% of women had a stable interpregnancy BMI (-1 to <1 kg/m2), 10.3% had BMI loss (>1 kg/m2), 34.3% had moderate BMI gain (1 to <3 kg/m2) and 14.8% had excess BMI gain (≥3 kg/m2). Women of younger age, Malay ethnicity and with higher BMI in the first pregnancy tended to experience excess BMI gain between their first two pregnancies.