A prospective study of 240 mothers observed until delivery reveals a high frequency of exposure to potential teratogens in the first trimester of pregnancy (mean exposure per mother, 3.7 potentially teratogenic agents). Although some exposures are unavoidable, the great majority of these exposures, including radiation and drugs, would be considered readily avoidable. The mean experience in the first trimester was 3.1 drug exposures per mother, which emphasizes not only that drug exposures are common, but that they are frequently multiple. Drugs used by the mothers were most often obtained by prescription. The significance of the role of teratogens in human malformations is not clearly established. However, until adequate information is available, it is appropriate to emphasize the magnitude of drug and other potentially teratogenic exposures to pregnant women and to enlist the support of physicians in the reduction of this exposure.