Maternal obesity in pregnancy increases the risk of adverse long-term health outcomes in both mother and offspring. A population-based cohort of prospectively collected routine antenatal healthcare data collected between January 2003 and September 2017 at University Hospital Southampton, UK was utilised to investigate the association between duration of interpregnancy interval between successive pregnancies and gain in maternal body mass index by the start of the next pregnancy. Records of 19362 women with two or more consecutive singleton live births were analysed. Two-thirds had gained weight when presenting to antenatal care for their subsequent pregnancy with 20% becoming overweight/obese. Compared to an interval of 24-35 months, an interval of 12-23 months was associated with lowest risk of weight gain (adjusted RR 0.91, 99% CI 0.87 to 0.95, p < 0.001) and ≥36 months with greatest risk (adjusted RR 1.11, 99% CI 1.07 to 1.15, p < 0.001) for the first to second pregnancy. This study shows that most multiparous women start their pregnancy with a higher weight than their previous one. An interval of 12-23 months is associated with the lowest risk of starting the second pregnancy with a higher body weight accounting for age. In countries with high prevalence of maternal obesity, birth spacing may merit exploration as a factor impacting on perinatal morbidity.