EFFECTS OF MATERNAL AGE, PARITY, AND SMOKING ON THE RISK OF STILLBIRTH

Objective To examine the effects of advanced maternal age, nulliparity , and smoking on risk of stillbirth as gestation advances, and to expl ore possible clinical mediators of these effects. Design A population based cohort study. Setting Sweden, 1983 to 1989. Subjects All singlet on pregnancies of 28 weeks gestation or greater in Nordic citizens at least 20 years old (n = 638242). Main outcome measures Crude and adjus ted risks of stillbirth; gestational age specific risks of stillbirth Results Older women (35 years or older), smokers, and nulliparas had e levated risks of stillbirth. The elevated stillbirth risk in smokers w as eliminated when women with intrauterine growth retardation, placent al abruption, and placenta previa were excluded from the analysis. How ever, the higher risks in older women and nulliparas persisted even wh en the analysis excluded women with hypertension, diabetes, placental complications, or growth retardation. Over the course of the third tri mester, the age related risk of stillbirth increased, the smoking rela ted risk decreased, and the higher risk in nulliparas showed no clear trend with gestational age. Conclusions The association between smokin g and stillbirth is explained entirely by the higher incidence of grow th retardation and placental complications in smokers. The clinical me diators of the associations of maternal age and parity with stillbirth remain unexplained. Gestational age is an important modifier of the e ffects of advanced maternal age and smoking on stillbirth risk.