Background. Smoking increases the risk of preterm birth. The present study was made to elucidate the relation of smoking to causes of very preterm birth.
Methods. In a case-control study on all very preterm births in two regions of Stockholm 1988–1992, prospectively collected data were extracted from antenatal and delivery records on smoking, other maternal characteristics, pregnancy complications, and causes of preterm birth. Cases were live single births with a gestational age of ≤32 weeks and 0 days, and controls were live singletons delivered at 37 weeks or later (n = 295, respectively). Odds ratios (ORs) with 95% confidence intervals (CIs) were calculated with logistic regression.
Results. Compared with non-smokers, adjusted ORs of very preterm birth among moderate smokers (1–9 cigarettes per day) and heavy smokers (≥10 cigarettes per day) were 1.4 (95% CI 0.8–2.4) and 2.9 (95% CI 1.5–5.7), respectively. Compared with non-smokers, risk of preterm labor was increased among moderate and heavy smokers [ORs 1.9 (95% CI 1.0–3.6) and 2.6 (95% CI 1.1–1.6), respectively]. These risks remained essentially unchanged in women without an identifiable cause of preterm labor (‘idiopathic preterm labor’). Smoking was also associated with dose-dependent increases in risks of preterm birth due to preterm premature rupture of membranes and late pregnancy bleedings. There was no association between smoking and risk of very preterm birth caused by hypertensive diseases.
Conclusions. Smoking increases the risk of very preterm birth caused by preterm labor (including idiopathic preterm labor), preterm premature rupture of membranes, and late pregnancy bleedings.