Objective To examine the change in maternal body mass index (BMI) between the first two deliveries and outcomes in the second delivery. Design Cohort study using electronic medical records. Setting and population Medical records of women with their first two consecutive deliveries between 2015 and 2020 at KK Women’s and Children’s Hospital, Singapore were retrieved. Methods Analysis was limited to women with BMI available for both pregnancies, which was standardised/adjusted to 12 weeks gestation. The difference between gestational-age-adjusted BMI in both pregnancies was calculated as the change in interpregnancy BMI. The risk ratios (RR) of pregnancy outcomes were estimated using modified Poisson regression models with confounder adjustment. Main outcome measures Low birthweight (<2.5 kg), high birthweight (≥4 kg), small-for-gestational-age, large-for-gestational-age, preterm delivery, gestational diabetes, elective and emergency caesarean deliveries. Results Of 6264 included women with a median interpregnancy interval of 1.44 years, 40.7% had a stable BMI change within +1 kg/m2, 10.3% lost >1 kg/m2, 34.3% gained 1-3 kg/m2 and 14.8% gained ≥3 kg/m2. Compared to women with stable BMI change, those with >1 kg/m2 loss had a higher risk of low birthweight delivery (RR 1.36; 95% confidence interval 1.02, 1.80), while those with 1-3 kg/m2 gain had higher risks of large-for-gestational-age birth (1.16; 1.03, 1.31), gestational diabetes (1.25; 1.06, 1.49) and emergency caesarean delivery (1.16; 1.03, 1.31); these risks were higher in those with ≥3 kg/m2 gain. Conclusion Our study demonstrated the importance of returning to pre-pregnancy weight and maintaining a stable interpregnancy BMI, to achieve better pregnancy outcomes.