title: A strong synergism of low birth weight and prenatal smoking on asthma in schoolchildren.
abstract: BACKGROUND
  Prenatal smoke exposure is associated with airway inflammation and asthma in children. It also increases the risk of low birth weight (LBW). LBW is associated with decreased lung function independently of smoking.
  
  
  OBJECTIVE
  To study the independent and joint effects of prenatal smoking and LBW on childhood asthma.
  
  
  METHODS
  In 1996, all children aged 7 to 8 years in 3 cities in northern Sweden were invited to an International Study of Asthma and Allergy in Childhood questionnaire survey. This study focused on the follow-up of children aged 11 to 12 years, in which 3389 children (96%) participated. A subset of 2121 children underwent skin-prick testing. Self-reported physician-diagnosed asthma has been clinically validated.
  
  
  RESULTS
  Mean birth weight was 3360 g in children exposed to prenatal smoking and 3571 g in nonexposed children (P < .001). The association of prenatal smoking with physician-diagnosed asthma was stronger in LBW children (risk ratio: 8.8 [95% confidence interval: 2.1-38]) than in normal birth weight children (risk ratio: 1.3 [95% confidence interval: 1.0-1.8]). LBW alone was not an independent predictor of asthma. These associations were similar in multivariate analysis, and the interaction term LBW × smoking was highly statistically significant.
  
  
  CONCLUSIONS
  There was a strong interaction of LBW and prenatal-smoking on the risk of physician-diagnosed asthma, which has not been demonstrated previously. This was consistently seen with adjustment for known risk factors, including allergic sensitization. Plausibly, airway inflammation from prenatal smoke exposure induces obstructive symptoms more easily in the underdeveloped airways of LBW children.
authors.affiliation: OLIN Studies, Department of Medicine, Sunderby Central Hospital of Norrbotten, Luleå, Sweden. anders.bjerg@lung.umu.se
authors.forename: Anders
authors.initials: A
authors.lastname: Bjerg
authors.forename: Linnea
authors.initials: L
authors.lastname: Hedman
authors.forename: Matthew
authors.initials: M
authors.lastname: Perzanowski
authors.forename: Bo
authors.initials: B
authors.lastname: Lundbäck
authors.forename: Eva
authors.initials: E
authors.lastname: Rönmark
mesh_terms: D001249:Asthma / Q000453:epidemiology / Q000209:etiology*; D002648:Child; D002675:Child, Preschool; D005260:Female; D005500:Follow-Up Studies; D006306:Health Surveys; D006801:Humans; D007223:Infant; D007230:Infant, Low Birth Weight; D007231:Infant, Newborn; D007428:Intradermal Tests; D008297:Male; D011247:Pregnancy; D011297:Prenatal Exposure Delayed Effects / Q000453:epidemiology / Q000209:etiology*; D012130:Respiratory Hypersensitivity / Q000453:epidemiology / Q000209:etiology; D012307:Risk Factors; D012907:Smoking / Q000009:adverse effects*; D013223:Statistics as Topic; D013548:Sweden