Prenatal diagnostic technology makes it possible to offer women the option of electively terminating pregnancies affected by birth defects. Excluding these pregnancies from epidemiologic studies may affect study results. We explored this effect using examples from the literature. We calculated the bias in the odds ratio caused by excluding prenatally diagnosed pregnancies when the exposure of interest is not correlated with the likelihood of terminating an affected pregnancy and when it is correlated with an increase or decrease in this likelihood. We assumed that control infants did not have birth defects. When the exposure is not associated with the likelihood of a pregnancy termination, studies excluding terminations suffer a loss of precision. When the exposure is associated with an increase or decrease in this likelihood, the odds ratios are biased toward or away from the null, respectively. The magnitude of the bias will vary according to characteristics of the study population such as the prevalence of the exposure and the frequency with which prenatal diagnosis and elective termination are used. Whenever possible, pregnancies terminated after prenatal diagnosis must be included in epidemiologic studies.