Parental cigarette smoking and the risk of spontaneous abortion

Although cigarette smoking is often considered a risk factor for spontaneous abortion, the epidemiologic literature is actually inconsistent. Therefore, the authors examined maternal and paternal smoking and maternal passive smoke exposure using data from a large case-control study of spontaneous abortion (626 cases and 1, 300 controls) conducted in Santa Clara County, California, in 1986 and 1987. No excess risk of spontaneous abortion was seen in the 1% of women who smoked an average of more than 20 cigarettes per day in the first trimester. Moderate smokers (11-20 cigarettes per day) had a slightly elevated crude odds ratio of 1.3 (95% confidence interval 0.9-1.9), which was close to unity after adjustment for covariates. Paternal smoking showed a slight crude elevation for moderate and heavy smoking, but no association after adjustment. In contrast, maternal exposure to environmental tobacco smoke for 1 hour or more per day was associated with spontaneous abortion, even after adjustment (odds ratio = 1.5, 95% confidence interval 1.2-1.9). For both maternal direct and environmental exposure, the association appeared to be stronger in second-trimester abortions. Several studies have found stronger associations of smoking with late versus early abortions, perhaps reflecting smoking-associated placental insufficiency and fetal hypoxia. Am J Epidemiol 1992; 135: 1394-1403