Stillbirths and infant deaths associated with maternal smoking among mothers aged > or =40 years: a population study.

We set out to estimate the association between smoking among pregnant women aged at least 40 years and pregnancy outcome by analyzing singleton live births in the United States between 1995 and 1997. The study group consisted of deliveries to mothers aged 40 years and older with two maternal age categories (20 to 29 and 30 to 39 years) as control. Although risks varied with maternal age, smoking was associated with a higher-than-expected risk for infant mortality in all maternal age categories. The highest rate of infant mortality associated with smoking after adjusting for confounding was among mothers aged 20 to 29 (hazard ratio [HR], 1.49; 95% confidence interval [CI], 1.28 to 1.75), while the lowest was among pregnant mothers in the 40 and above age category (HR, 1.03; 95% CI, 0.87 to 1.23). In utero fetal demise was highest among older smoking mothers (>/=40 years) and declined with decreasing age (p for trend <0.0001). In conclusion, the relationship between maternal smoking and pregnancy outcomes is modified by the age of the mother.