Rationale Previous studies have raised concern that women with asthma have increased risks of adverse obstetric and paediatric complications, but have generally been underpowered. Objectives To quantify risks of major adverse pregnancy outcomes and obstetric complications in women with and without asthma. Methods We extracted information on 281,019 pregnancies from The Health Improvement Network database, between 1988 and 2004. We analysed the data using logistic regression. Results In 37,585 pregnancies of women with asthma compared with 243,434 pregnancies of women without asthma, risks of stillbirth and therapeutic abortion were similar, however, the risk of miscarriage was slightly higher (OR=1.10, 95%Confidence Interval(CI) 1.06-1.13). Risks of most obstetric complications (placental abruption, placental insufficiency, placenta praevia, pre-eclampsia, hypertension, gestational diabetes, thyroid disorders in pregnancy, and assisted delivery) were not higher in pregnancies of women with asthma compared to those without, with the exception of increases in antepartum (OR=1.20, 95%CI 1.08-1.34) or postpartum (OR=1.38, 95%CI 1.21-1.57) haemorrhage, anaemia (OR=1.06, 95%CI 1.01-1.12), depression (OR=1.52, 95%CI 1.36-1.69) and caesarean section (OR=1.11, 95%CI 1.07-1.16). Risks of miscarriage, depression and caesarean section increased moderately in women with more severe asthma and previous asthma exacerbations. Conclusion We found some increased risks in women with asthma that need to be considered in future, however, our results indicate that women with asthma have similar reproductive risks compared with women without asthma in the general population, for most of the range of outcomes studied.