The Impact on Infant Birth Weight and Gestational Age of Cotinine-Validated Smoking Reduction During Pregnancy

Abstract

Maternal smoking and low-birth weight are associated in a dose-dependent fashion. Smoking cessation trials have documented increases in mean birth weight of up to 264 gm due to smoking cessation. To date, no study has documented an association between increased infant birth weight and cotinine-validated smoking reduction in pregnancy. This was a prospective, randomized clinical trial conducted from 1986 through 1991 in the maternity clinics of a major southern city. The experimental group received a multi-faceted smoking cessation program, including a counseling session, a self-directed workbook, reinforcement by clinic staff, and the establishment of social support systems. The control group received written information and verbal advice to quit. In the experimental group 14.3 per cent women were able to stop smoking compared to 8.4 per cent of women in the routine care group. A mother's self-report of smoking cessation was validated by a salivary cotinine test at the beginning, midportion, and end of pregnancy. Based on the salivary cotinine levels, women were assigned to one of three groups: quitters who stopped smoking completely, reducers who continued to smoke but at lower levels, and no-changers who did not change smoking behavior. Reducers had either a 60-ng/ml reduction from baseline, if they were heavy smokers (baseline cotinine greater than 100 ng/ml), or a 20-ng/ml reduction, if they were light smokers (baseline cotinine level less than 100 ng/ml). A 20-ng/ml reduction in cotinine level is approximately equal to a reduction of three cigarettes smoked per day. Quitters had the highest mean birth weight (3371 ± 581 gm), followed by reducers (3121 ± 651 gm), and no-changers (3043 ± 587 gm) (Table 1). The mean birth weight of infants born to quitters, adjusted by mother's age, race, height, weight at baseline, and gestational age at delivery, was 241 gm heavier than that among the no-changers. White reducers had greater increases in birth weight than did black reducers. Smoking cessation increased infant gestational age at delivery by 1 week, whereas smoking reduction showed no changes in gestational age at delivery. Although smoking cessation remains the primary objective for pregnant smokers, this article shows that smoking reduction is associated with favorable outcomes for women who cannot or will not quit. The increase in birth weight due to smoking cessation and smoking reduction documented in this study has significant clinical and public health implications.