Objective: To examine the effects of advanced maternal age, nulliparity, and smoking on risk of stillbirth as gestation advances, and to explore possible clinical mediators of these effects.
Design: A population based cohort study.
Setting: Sweden, 1983 to 1989.
Subjects: All singleton pregnancies of 28 weeks gestation or greater in Nordic citizens at least 20 years old (n = 638,242).
Main outcome measures: Crude and adjusted risks of stillbirth; gestational age specific risks of stillbirth.
Results: Older women (35 years or older), smokers, and nulliparas had elevated risks of stillbirth. The elevated stillbirth risk in smokers was eliminated when women with intrauterine growth retardation, placental abruption, and placenta previa were excluded from the analysis. However, the higher risks in older women and nulliparas persisted even when the analysis excluded women with hypertension, diabetes, placental complications, or growth retardation. Over the course of the third trimester, the age related risk of stillbirth increased, the smoking related risk decreased, and the higher risk in nulliparas showed no clear trend with gestational age.
Conclusions: The association between smoking and stillbirth is explained entirely by the higher incidence of growth retardation and placental complications in smokers. The clinical mediators of the associations of maternal age and parity with stillbirth remain unexplained. Gestational age is an important modifier of the effects of advanced maternal age and smoking on stillbirth risk.