The worldwide prevalence of asthma is increasin, and with 3-12 per cent of pregnant women affected by asthma, it is the most common chronic disease in pregnancy. Asthma is not consistently affected by pregnancy and it is reported that, during pregnancy, the severity of asthma remains stable in one-third of women, worsens in another third and improves in the remaining third. However, prospective studies show that exacerbations of asthma are more likely to occur in women with severe asthma than mild asthma and that most episodes occur between 24 and 36 weeks of pregnancy.