Maternal smoking and Down syndrome: The confounding effect of maternal age

Inconsistent results have been reported from studies evaluating the associa tion of maternal smoking with birth of a Down syndrome child, Control of kn own risk factors, particularly maternal age, has also varied across studies . By using a population-based case-control design (775 Down syndrome cases and 7,750 normal controls) and Washington State birth record data for 1984- 1994, the authors examined this hypothesized association and found a crude odds ratio of 0.80 (95% confidence interval 0.65-0.98). Controlling for bro ad categories of maternal age (<35 years, greater than or equal to 35 years ), as described in prior studies, resulted in a negative association (odds ratio = 0.87, 95% confidence interval 0.71-1.07). However, controlling for exact year of maternal age in conjunction with race and parity resulted in no association (odds ratio = 1.00, 95% confidence interval 0.82-1.24), In t his study, the prevalence of Down syndrome births increased with increasing maternal age, whereas among controls the reported prevalence of smoking du ring pregnancy decreased with increasing maternal age. There is a substanti al potential for residual confounding by maternal age in studies of materna l smoking and Down syndrome. After adequately controlling for maternal age in this study, the authors found no clear relation between maternal smoking and the risk of Down syndrome.