THE RELATIONSHIP OF PARENTS' CIGARETTE SMOKING TO OUTCOME OF PREGNANCY—IMPLICATIONS AS TO THE PROBLEM OF INFERRING CAUSATION FROM OBSERVED ASSOCIATIONS1

Abstract

The relationship of parental smoking to outcome of pregnancy was studied in 9793 white and 3290 black women who were interviewed early in pregnancy on a variety of medical genetic environmental and behavioral variables. An increase in incidence of low-birth-weight infants was confirmed (P < .00001). However a number of paradoxical findings were observed which raised doubts as to causation. No increase in neonatal mortality was noted; rather the neonatal mortality rate and the risk of congenital abnormality of low-birth-weight infants were considerably lower for smoking than for nonsmoking mothers (P < .005 for whites and .05 < P < .06 for blacks). These more-favorable-than-expected results cannot be explained by differences in gestational age nor by a displacement hypothesis. Among other findings that could not be easily explained were: 1) that the healthiest low-birth-weight babies were found where the mother smoked and the father did not; 2) that the most vulnerable infants were produced when the mother did not smoke but the father did; 3) and that great differences obtained in mode-of-life characteristics between smokers and nonsmokers.