The course of asthma during pregnancy is variable. When the total pregnant asthmatic population is considered, one-third improve, one-third are unchanged, and one-third experience increased symptoms. The course of asthma may be influenced by the many physiologic changes during pregnancy as well as the severity of the pre-existing asthma. Prospective studies have found that severe asthmatics have exacerbations during pregnancy more often than mild asthmatics. The first trimester and the last month of pregnancy are relatively free of exacerbations. The second and third trimesters have more potential for increased asthma symptoms and the need for medications. Change in allergic nasal symptoms may precede similar changes in asthma during pregnancy. The course of asthma during labor and delivery is worsened in only 10% of patients, and their problems are usually mild. Postpartum most women return to their prepregnant state within 3 mo. Physiologic changes in pregnancy affect the pulmonary function with a decrease in functional residual capacity (FRC) and 50% increase in minute ventilation. This change results in hyperventilation in 60-70% of normal pregnancies and a sensation of dyspnea that can be distressing to an asthmatic. There is also nasal congestion in 22-72% of pregnancies beginning in the second trimester, which can also affect asthma. The gastrointestinal system is affected in pregnancy with one-third of women having gastroesophageal reflex disease (GERD). Undertreatment of asthma during pregnancy remains a problem in emergency departments. With awareness of the patient's prepregnant state, careful monitoring during pregnancy will prevent serious exacerbations and complications.