Asthma is the most common respiratory problem in women of reproductive age and is the most common pulmonary complication of pregnancy. Medical therapy for asthma during pregnancy raises concern that the medications the mother must take will adversely affect the developing embryo. Counseling any woman who must undergo medical treatment during a pregnancy requires determining whether her medication is associated with an increased teratogenic risk, and if so, whether the medication can be safely avoided during pregnancy, used less frequently, or replaced by a safer alternative if treatment cannot be avoided. Adequate pulmonary function during pregnancy is critical, and therefore the majority of pregnant asthma patients must continue to take their medications. Additional drugs are sometimes required during acute episodes. Determining the teratogenic risk associated with a particular asthma medication requires careful interpretation of data from animal and clinical studies and an understanding of the basic principles of teratology. This article reviews these principles of teratology and how information derived from teratology studies can be used to evaluate teratogenic risk in the pregnant asthmatic patient.