Effect of passive smoking on birth-weight.

500 consecutive Danish women who had full-term babies were interviewed on the third or fourth day post partum and asked about smoking in all household members. Exposure to smoking by the mother was found to reduce birth-weight, and indirect or passive exposure to smoking by the father had nearly as large (66%) an effect. On average, birth-weight was reduced by 120 g per pack of cigarettes (or cigar/pipe equivalent) smoked per day by the father. This relation remained statistically significant after controlling for mother's age, parity, alcohol and tobacco consumption during pregnancy, illness during pregnancy, and social class and sex of the baby. The effect of passive smoking was greatest in the lower social classes.