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.