DOWNS-SYNDROME AND MATERNAL SMOKING IN EARLY-PREGNANCY

Several previous studies have suggested that maternal smoking is assoc iated with a decreased incidence of trisomy 21. By using the Swedish h ealth registries, 1,321 infants with Down's syndrome (DS) were selecte d among 1,117,021 infants born in 1983-1993 with known smoking exposur e in early pregnancy. No association between maternal smoking and all cases of DS was found [age-adjusted odds ratio (OR) for maternal smoki ng: 0.98; 95% confidence interval (CI): 0.86-1.11], but heterogeneity over strata existed. A slightly decreased OR (0.91; 95% CI: 0.72-1.15) for any maternal smoking was indicated among primiparas, but among mu ltiparas, no effect of smoking on the incidence of DS could be detecte d (OR: 1.01; 95% CI: 0.87-1.17). The difference between the OR for smo king greater than or equal to 10 cigarettes per day among primiparas ( OR: 0.59; 95% CI: 0.38-0.90) and multiparas (OR: 1.06; 95% CI: 0.86-1. 31) was statistically significant. If not due to statistical fluctuati on, the findings indicate that no direct effect of smoking on DS risk exists but the association observed in primiparas is due to covarying factors. (C) 1997 Wiley-Liss, Inc.