Objective: To evaluate the association between smoking during pregnancy and preterm birth.
Design: A follow up study.
Setting: Department of Gynaecology and Obstetrics, Aarhus University Hospital, Denmark.
Participants: Four thousand one hundred and eleven nulliparous women with singleton pregnancies who returned questionnaires about smoking habits at 16 weeks of gestation.
Results: The overall rate of preterm delivery was 4.3%. Smokers had a 40% higher risk of preterm birth compared with nonsmokers. A dose response relationship was found between smoking and risk of preterm birth. Adjustment for women's height, prepregnancy weight, age of the mother, marital status, education, occupational status, and alcohol intake did not change the results. Among women with an intake of less than 400 mg of caffeine per day no difference in the risk of preterm birth between smokers and nonsmokers was found. However, among women with an intake of more than 400 mg of caffeine per day, the risk of preterm birth was increased almost threefold among smokers compared with nonsmokers. Furthermore, among women with a high intake of caffeine a dose-response relationship was found; women smoking one to five cigarettes per day had no increased risk of preterm birth compared with nonsmokers with the same intake of caffeine, women smoking six to ten cigarettes per day had almost three times higher risk of preterm birth, and women smoking more than 10 cigarettes per day had almost five times higher risk of preterm birth compared with nonsmokers with the same intake of caffeine.
Conclusions: Smoking increases the risk of preterm birth. The association between smoking and preterm birth was only present among women with a high intake of caffeine. However, whether smoking alone influences the risk of preterm birth among heavy consumers of caffeine needs further investigation.