The relationship of parents' cigarette smoking to outcome of pregnancy--implications as to the problem of inferring causation from observed associations.

The relationship of parents' cigarette smoking to outcome of pregnancy-implications as to the problem of inferring causation from observed associations. Amer J Epidem 1971;93:443-456. Nearly 10,000 white and more than 3000 black women were interviewed early in pregnancy on a variety of medical, genetic, environmental, and behavior variables. The increase in the incidence of low-birth-weight among infants of smoking mothers was confirmed. However, a number of paradoxical findings were observed which raise doubts as to causation. Thus, no increase in neonatal mortality was noted. Rather, the neonatal mortality rate and the risk of congenital anomalies of low-birthweight infants were considerably lower for smoking than for nonsmoking mothers. These favorable results cannot be explained by differences in gestational age, nor does a "displacement" hypothesis appear reasonable. Among other findings which could not easily be explained: The healthiest low-birth-weight infants were found for couples where the wife smoked and her husband did not smoke; the most vulnerable were produced by couples where the wife did not smoke and the husband smoked. There were great differences in mode-of-life characteristics between smokers and nonsmokers. The latter were more likely to use contraceptive methods, to plan the baby, less likely to drink coffee and hard liquor, and in general appeared to live at a much slower and moderate pace than the smokers. Most puzzling difference is that of age at menarche, which was lower for smoking mothers. These paradoxical findings raise doubts and argue against the proposition that cigarette smoking acts as an exogenous factor which interferes with intrauterine development of the fetus.