Twin pairs, smoking in pregnancy and perinatal mortality.

Twins born in the province of Quebec during the first six months of 1972 were ascertained by reviewing all live births and perinatal death certificates. A total of 852 twins, constituting 426 pairs, were included in the study. Smoking habits of the mother during pregnancy were recorded at the hospital of birth for 88.5% of the pairs. The risk of at least one perinatal death was significantly higher for twin pairs born to smoking mothers. The Mantel-Haenszel test statistic controlling for sex of pairs, parity (0,1+), maternal age (< 25,25+ years) and years of schooling was 4.224 (p = 0.04). The standardized risk ratio for smoking versus nonsmoking mothers that a pair will have at least one death was 1.77 (95% confidence limits: 1.03-3.04). As expected, the relative risk of perinatal death associated with smoking appears to be higher for twins than for singletons.