Spontaneous abortion as a screening device. The effect of fetal survival on the incidence of birth defects.

To make sound inferences about the distribution, causes and risks of congenital anomalies requires an understanding of selective survival among fetuses. Studies of spontaneous abortions are therefore advocated. A simple model is proposed of the distribution, among spontaneous abortions and births, of conceptuses with and without anomalies. The resulting equation accomodates both maternal and fetal factors. Interaction between maternal and fetal factors is hypothesized, in the form of a maternal device for screening out anomalous conceptuses. A number of the known facts about congenital anomalies are shown to be plausibly interpreted in terms of the proposed model. Finally, some clinical and epidemiologic implications of the model for the study of spontaneous abortions are pointed out.