The effects of maternal smoking on infant birth weight.

A prospective cohort study was designed to examine the independent effects of maternal smoking on infant birth weight adjusted for length of gestation. Obstetrical patients were enrolled from four university based family practice clinics; 772 mother-infant pairs were included in the final analyses. A multiple regression analysis found 53% of the variance in infant birth weight to be explained by length of gestation, race, pre-pregnancy weight, weight gain, gender of the infant, highest systolic blood pressure, and smoking. After adjusting for the effects of other independent variables, women who were self-reported smokers delivered infants who weighed 141.8 g less, on the average, than infants of nonsmoking mothers. Women who smoked fewer than 10, 10-19, or 20+ cigarettes per day delivered infants weighing 96 g, 183 g, and 200 g less, respectively, than infants born to nonsmokers, a clear dose-response effect among relatively low-risk obstetrical patients in a primary care setting. Multiple regression analysis within each smoking group found the effect of length of gestation upon infant birth weight to be markedly reduced among the heavy smokers; this indicates a strong association between heavy (one pack a day) smoking and infant birth weight adjusted for length of gestation.