Background/aim: The effects of advanced maternal age and smoking in pregnancy on fetal survival have previously been reported. However, whether advanced maternal age modifies the relationship between smoking in pregnancy and intrapartum stillbirth remains unknown. We therefore set out to determine the impact of advanced maternal age (> or =35 years) on the association between smoking during pregnancy and intrapartum stillbirth by employing retrospective analysis of birth registry data.
Methods: We used a cohort of singleton births in Missouri from 1978 through 1997 (N = 1,436,628) to compute the risk of total, antepartum, and intrapartum stillbirth in smoking mothers. We categorized mothers into two age groups: "younger" (<35 years), and "older" (> or =35 years). Non-smoking mothers age <35 years were the referent category. Cox regression models were used to generate independent measures of association between intrauterine tobacco exposure and the risk of total, antepartum, and intrapartum stillbirth in each age group.
Results: A total of 5,772 counts of stillbirth were identified, yielding a stillbirth rate of 4.0 per 1,000. Approximately 33% (N = 1,900) occurred among older smokers resulting in a stillbirth rate of 9.1 per 1,000. The probability of intrapartum stillbirth was greatest among older smokers, followed by younger smokers and lowest among younger non-smokers (P < 0.01). As compared to non-smoking younger gravidas, younger smoking mothers had a 30% greater likelihood for both antepartum and intrapartum stillbirth (adjusted hazard ratio [95% confidence interval]: 1.3 [1.2-1.4] and 1.3 [1.2-1.5], respectively). Among older smokers the risk for intrapartum stillbirth was three times that of the referent group (adjusted hazard ratio: 3.2, 95% confidence interval: 2.2-4.5).
Conclusions: The risk of intrapartum stillbirth associated with smoking in pregnancy is potentiated by the age of the mother. This information will help policy makers develop targeted smoking cessation campaigns and positively impact quit rates in older mothers.