Background: Asthma now affects up to 10% of pregnant women in high income countries and international prevalence is rising. It is already one of the commonest chronic diseases that can complicate pregnancy and previous studies have raised concern that women with asthma have increased pregnancy risks. Precise estimates of the magnitude of these risks and the extent to which they may differ by asthma severity and asthma exacerbation rates, have not been determined. Aim: The overall aim of this thesis was to investigate the impact of asthma and asthma therapies on pregnancy and perinatal outcomes in the general female population. Methods: The Health Improvement Network primary care database from the United Kingdom was used to develop a dataset of women matched to their liveborn children and all data on these pregnancies and on pregnancies ending in stillbirth, miscarriage or therapeutic abortion, were extracted for analysis. Three separate studies were carried out using the developed dataset. First, a cohort design was used to compare fertility rates of women with and without asthma or other allergic disease. Secondly, a cross-sectional design was used to compare risks of adverse pregnancy outcomes and obstetric complications in women with and without asthma. Thirdly, a case-control design was used to compare the risk of congenital malformation in children born to women with and without asthma, and to assess whether asthma medications are teratogenic. Results: A study population of 1,059,246 women was obtained and pregnancies ending in 268,601 matched live births, 986 stillbirths, 35,272 miscarriages and 37,118 therapeutic abortions were identified. Women with asthma or other allergic disease had similar fertility rates (live births per 1,000 person-years) to women in the general population. Women with asthma also had a similar risk of pregnancy ending in stillbirth (Odds Ratio (OR)=1.04, 95% confidence interval (CI) 0.86-1.24)) or therapeutic abortion (OR=0.95, 95%CI 0.92-0.99), but had a small relative increase in risk of pregnancy ending in miscarriage (OR=1.10, 95%CI 1.06-1.13), compared with women without asthma. Risks of most obstetric complications were similar in women with and without asthma, regardless of asthma severity or acute exacerbations, with the exception of increased risks of antepartum haemorrhage (OR=1.20, 95%CI 1.08-1.34), postpartum haemorrhage (OR=1.38, 95%CI 1.21-1.57), depression in pregnancy (OR=1.52, 95%CI 1.36-1.69), caesarean section delivery (OR=1.11, 95%CI 1.07-1.16), preterm delivery (OR=1.15, 95%CI 1.06-1.24) and low birth weight (OR=1.18, 95%CI 1.05-1.32) in their offspring. Compared with children born to mothers without asthma, children born to mothers with asthma had a small increased risk of major congenital malformation (OR=1.10, 95%CI 1.01-1.20), however, this was not found for mothers with currently treated asthma (OR=1.06, 95%CI 0.94-1.20). Gestational exposure to asthma medications was safe apart from cromones which may increase the risk of musculoskeletal malformation. Conclusions: These findings indicate that women with asthma do not have substantially increased risks associated with pregnancy or with perinatal outcomes. Treatment with asthma medications before pregnancy and during gestation also appear to be safe for the mother and for the unborn child, providing support for the current practice of optimal pharmacological management of asthma in women of childbearing age.