Advanced maternal age, nulliparity, and cigarette smoking are established risk factors for stillbirth. We used data from the Medical Birth Register of Sweden, 1983-1989, to examine the interactions between these risk factors and gestational age, and to explore possible clinical mediators of these relationships. All singleton pregnancies of at least 28 weeks gestation to nordic citizens at least 20 years old were included in the analysis. We found significant interactions of both maternal age and smoking with gestational age, and these interactions had different patterns: over the course of the third trimester the age-related risk of stillbirth increased, whereas the smoking related risk decreased. The higher risk in nulliparas remained constant. The elevated stillbirth risk in smokers appeared to be explained entirely by the higher incidence of intrauterine growth retardation and placental complications in smokers. However, the increased risk in older women and nulliparas was not explained by any of the pregnancy complications examined.