The influence of maternal smoking habits on the risk of subsequent stillbirth: is there a causal relation?

OBJECTIVE Maternal smoking has previously been associated with risk of stillbirth. If women who quit smoking reduce their risk of stillbirth, the hypothesis of a causal association would be supported. DESIGN Prospective cohort study. SETTING Nationwide study in Sweden. POPULATION All primiparous women who delivered their first and second consecutive single births between 1983 and 2001, giving a total number of 526,691 women. METHOD A population-based Swedish study with data from the Medical Birth Registry, the Immigration Registry and the Education Registry. Logistic regression analyses were used to estimate odds ratios, using 95% confidence intervals. MAIN OUTCOME MEASURE Stillbirth in the second pregnancy. RESULTS Compared with nonsmokers in both pregnancies, women who smoked during the first pregnancy but not during the second do not have an increased risk of stillbirth (OR 1.02; 95% CI 0.79-1.30), while corresponding risk among women who smoked during both pregnancies was 1.35 (95% CI 1.15-1.58). CONCLUSION The result supports that maternal smoking during pregnancy is causally associated with stillbirth risk. Smoking is a preventable cause of stillbirth, and smoking interventions is an important issue in antenatal care.