title: Smoking during pregnancy increases the risk of preterm births among parous women.
abstract: BACKGROUND
  We wanted to study a possible association between smoking during pregnancy and preterm birth.
  
  
  METHODS
  A population based prospective study was performed among 7,236 consecutive pregnancies over seven years delivered at one birth clinic. Smoking habits were obtained at a routine ultrasound examination around 18 gestational weeks, and preterm birth was defined as delivery of a child between 24 and 37 gestational weeks. We estimated relative risk (odds ratio) between smoking, possible confounding variables and preterm birth using multiple logistic regression analysis.
  
  
  RESULTS
  Overall, 1,769 (25%) women said that they smoked every day during their pregnancy. The smoking women had an increased risk of preterm delivery compared with non-smokers (odds ratio 1.53, 95% confidence interval 1.24, 1.89). However, this association was only demonstrated among the multiparous women (odds ratio 1.88, 95% confidence interval 1.39, 2.54). The association was statistically significant after adjusting for maternal age, diabetes, cervical conization, preeclampsia, placental abruption and placenta previa in a multivariate analysis, and we observed a dose-response effect of maternal smoking on preterm birth among multiparous women.
  
  
  CONCLUSIONS
  For multiparous women smoking during pregnancy increases the risk of preterm delivery, and there seems to be a dose-response effect of smoking.
authors.affiliation: Department of Obstetrics and Gynecology, Lillehammer County Hospital, Norway.
authors.forename: T
authors.initials: T
authors.lastname: Kolås
authors.forename: J
authors.initials: J
authors.lastname: Nakling
authors.forename: K A
authors.initials: KA
authors.lastname: Salvesen
mesh_terms: D000293:Adolescent; D000328:Adult; D005260:Female; D006801:Humans; D007752:Obstetric Labor, Premature / Q000209:etiology*; D010298:Parity; D011247:Pregnancy; D011248:Pregnancy Complications / Q000209:etiology*; D011256:Pregnancy Outcome; D011446:Prospective Studies; D012044:Regression Analysis; D012307:Risk Factors; D012907:Smoking / Q000009:adverse effects*