Smoking habits and spontaneous abortion.

Smoking habits have been compared in three samples of pregnancies: (1) spontaneous abortions (n = 610); (2) induced abortions (n = 800); and (3) deliveries (n = 1337). The variables studied were, besides smoking habits, day of LMP, outcome of earlier pregnancies, maternal age, and, for the delivery sample, also diagnoses of mother and child, gestational length, sex, and birthweight. A statistical analysis of the association between smoking and the risk of having a spontaneous abortion was made. The comparisons were made with all types of intra-uterine pregnancies but spontaneous abortions, e.g., deliveries and induced abortions. The effects and consequences of that are discussed. The smoking rates according to pregnancy outcome differ among the samples. In the induced abortion sample 58% smoked compared with 50% in the spontaneous abortion sample and 44% in the delivery sample. The well-known effect of smoking on gestational length and birthweight was shown. No significant effect of smoking on the miscarriage risk was seen. The only trend was the opposite. Possible explanations for this are discussed.