Aims
Maternal overweight and obesity in pregnancy are known to increase the risk of a range of complications and adverse pregnancy outcomes. This study estimates the population‐level contribution of maternal overweight and obesity to adverse pregnancy outcomes.

Methods
Data derived from the Australian Capital Territory (ACT) Maternal and Perinatal Data Collection were analysed. A total of 24 161 women who had a singleton birth in 2009–2015, with maternal weight and height information available, were included. In this study, the association between risk factors and outcomes was investigated using multilevel regression modelling. Based on model predictions under various hypothetical maternal weight scenarios, the number and proportion of adverse perinatal outcomes that could be potentially prevented were estimated.

Results
Maternal overweight and obesity were associated with increased risks of gestational diabetes mellitus (GDM), pre‐eclampsia, caesarean delivery, preterm birth (PTB), large for gestational age (LGA) and admission to the special care nursery or neonatal intensive care unit (SCN/NICU). The estimated proportions of adverse pregnancy outcomes attributable to overweight and obesity in pregnancy are 29.3% for GDM, 36.2% for pre‐eclampsia, 15.5% for caesarean delivery, 21.6% for longer antenatal stay in hospital (≥2 days), 16.3% for extreme PTB, 25.2% for LGA and 6.5% for SCN/NICU admission.

Conclusions
Maternal overweight and obesity contribute to a large proportion of obstetric complications and adverse outcomes in the ACT. Effective intervention strategies to reduce the prevalence of overweight and obesity in pregnant women could have significant beneficial effects on pregnancy outcomes.