Does multiple gestation impact birthweight deficit from smoking?

To test the hypothesis that the combined birthweight deficit from smoking in twin gestation is significantly greater than previously described for singletons.Retrospective cohort study of White/Black live-born twins, 32 to 40 weeks of gestation, 1990 to 2005, Kansas City, MO. Data were analyzed by t-test, χ(2), linear and linear regression analyses.The cohort comprised 1190 twin pairs, 58% Whites and 55% ≥37 weeks. Smoking rate was 14.5% and similar by race (14.2% White vs 14.8%, P=0.772). The aggregate birthweight of twins of nonsmokers was greater, 5116 g ± 25 g vs 4870 g ± 59 g, P<0.001 (mean±s.d.) and the difference was 264 g (95% confidence interval, 156 to 372 g) after adjusting for other variables in linear regression analyses.Smoking-associated growth restriction in twins is quantitatively similar to that of singletons. This finding is suggestive of dissipation or a ceiling effect to the growth-restricting effect of cigarette smoking.