Abstract and Introduction
We conducted a systematic review and meta-analysis to characterize the relationship between smoking and miscarriage. We searched the PubMed database (1956–August 31, 2011) using keywords and conducted manual reference searches of included articles and reports of the US Surgeon General. The full text of 1,706 articles was reviewed, and 98 articles that examined the association between active or passive smoking and miscarriage were included in the meta-analysis. Data were abstracted by 2 reviewers. Any active smoking was associated with increased risk of miscarriage (summary relative risk ratio = 1.23, 95% confidence interval (CI): 1.16, 1.30; n = 50 studies), and this risk was greater when the smoking exposure was specifically defined as during the pregnancy in which miscarriage risk was measured (summary relative risk ratio = 1.32, 95% CI: 1.21, 1.44; n = 25 studies). The risk of miscarriage increased with the amount smoked (1% increase in relative risk per cigarette smoked per day). Secondhand smoke exposure during pregnancy increased the risk of miscarriage by 11% (95% CI: 0.95, 1.31; n = 17 studies). Biases in study publication, design, and analysis did not significantly affect the results. This finding strengthens the evidence that women should not smoke while pregnant, and all women of reproductive age should be warned that smoking increases the risk of miscarriage.
Smoking during pregnancy causes low birthweight, placental abruption, and sudden infant death syndrome.[1,2] Nonetheless, 14% of pregnant women and 23% of women of reproductive age report being smokers, with secondhand smoke exposure even more prevalent, at 37% of pregnant women.[3–5] Miscarriage, or loss of the fetus before it is viable, is the most common complication of pregnancy, occurring in 12%–26% of recognized pregnancies. Although many studies have addressed the association between miscarriage and smoking, the evidence has been considered inconclusive. The US Surgeon General's most recent conclusion, from the 2004 report, classified the evidence as suggestive but not sufficient to infer causation, and the most recent edition of the authoritative textbook, Williams Obstetrics, also describes the lack of consistency. Most miscarriages end early in pregnancy, during an interval over which a woman might not yet have learned of being pregnant or begun prenatal care. The benefits of quitting smoking early in pregnancy include increased birthweight and a lower risk of preterm birth, but these effects would never manifest for a woman who quits smoking but also miscarries in the first trimester. Thus, a more complete understanding of the relationship between smoking and miscarriage is important for preconception counseling and public health programs for women of reproductive age. This systematic review and meta-analysis focuses on the association of smoking (active and passive) with miscarriage.
Am J Epidemiol. 2014;179(7):807-823. © 2014 Oxford University Press