Objective: The object was to examine the effects of smoking on spontaneous and induced very and moderately preterm birth (< or = 32 and 33-36 completed weeks' gestation, respectively).
Study design: Live singleton births in Sweden, 1991-1993 (n = 311,977), were examined.
Results: Risk of preterm birth consistently increased with amount smoked. Smoking was most heavily associated with increased risks of very preterm birth and spontaneous preterm birth. The highest impact of smoking was seen on risk of spontaneous very preterm birth among women who smoked at least 10 cigarettes/d (odds ratio 1.7). The smoking-related risks of preterm birth remained essentially unchanged after excluding pregnancies with smoking-associated pregnancy complications.
Conclusions: There is a dose-related impact of smoking on risk of preterm birth. The fact that the smoking-related risk of spontaneous preterm birth is more pronounced than that of induced preterm birth suggests that smoking is associated with spontaneous preterm labor.