Effect of prenatal diagnosis on epidemiologic studies of birth defects

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 e xplored this effect using examples from the literature. We calculated the b ias in the odds ratio caused by excluding prenatally diagnosed pregnancies when the exposure of interest is not correlated with the likelihood of term inating 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 pre cision. When the exposure is associated with an increase or decrease in thi s likelihood, the odds ratios are biased toward or away from the null, resp ectively. The magnitude of the bias will vary according to characteristics of the study population such as the prevalence of the exposure and the freq uency with which prenatal diagnosis and elective termination are used. When ever possible, pregnancies terminated after prenatal diagnosis must be incl uded in epidemiologic studies.