Maternal smoking during lactation: relation to infant size at one year of age.

This study was conducted to test the hypothesis that breast-fed infants of smokers are smaller in size at 1 year of age than breast-fed infants of nonsmokers. Three groups of infants were selected from all singletons born to women who were seen for prenatal care in their 6th month of pregnancy at a health maintenance organization in Seattle, Washington, between January 1982 and April 1983. Breast-fed infants of smokers (n = 74) were compared with breast-fed infants of nonsmokers (n = 195) and with bottle-fed infants of smokers (n = 64). Mothers were interviewed at 1 and 3 months after delivery; both the mother and the infant were seen at 1 year. Among breast feeders, smokers' infants were twice as likely as nonsmokers' infants to have body mass more than 1 standard deviation above the mean (relative risk = 2.04, 95% confidence interval 1.15-3.61). This relation persisted after control for gestational age and weight at birth, length of lactation, mother's size and diet, exposure to other drugs in breast milk, and all other variables measured in this study. Every 10 cigarettes smoked while breast feeding predicted an additional 3% infant body mass at 1 year. In summary, breast-fed infants of smokers in this study gained more weight after birth than the other two groups; at 1 year of age, they were heavier and had significantly higher body mass. Reasons for this paradoxical finding are explored.