Outcome data (birth weight) were obtained from maternal postpartum interviews when available. For infants of women not returning for a postpartum visit, birth weight was obtained from birth certificate records.

We used two different measures of tobacco exposure: self-reported cigarettes smoked per day, and urine cotinine concentration. We included only those women who had at least one self-reported estimate of the number of cigarettes smoked per day and a corresponding measure of urine cotinine obtained in the third trimester.