Passive and active maternal smoking as measured by serum cotinine: the effect on birthweight.

To determine how maternal exposure to environmental tobacco smoke affects birthweight, maternal sera obtained from 3529 pregnant women around 27 weeks gestation were analyzed for cotinine, a metabolite of nicotine. Based on cotinine levels, nonsmokers were divided into those exposed to environmental tobacco smoke (2-10 ng/mL) and those unexposed (< 2 ng/mL), and smokers were divided into tertiles. Compared with unexposed nonsmokers' infants, infants of exposed nonsmokers averaged 45 g less (P = .28) after adjustment for confounders, and smokers' infants averaged 78, 191, and 233 g less for the first, second, and third cotinine tertiles, respectively. Birthweight decreased 1 g for every nanogram per milliliter of cotinine increase (P < .001).