title: Preterm delivery and asthma: a systematic review and meta-analysis.
abstract: BACKGROUND
  Accumulating evidence suggests that reduced duration of pregnancy predicts increased risk of asthma, but the studies published have been inconsistent.
  
  
  OBJECTIVE
  We sought to synthesize the evidence on the relation between preterm delivery and the risk of asthma later in life and to assess differences between the studies as potential sources for heterogeneity of the results.
  
  
  METHODS
  We conducted a MEDLINE search (until the end of May 2005). The outcome was asthma. The determinant of interest was preterm delivery defined as a gestational age of less than 37 weeks.
  
  
  RESULTS
  We identified 19 articles that provided estimates for the meta-analysis. The summary effect estimates for asthma (fixed-effects odds ratio, 1.074 [95% CI, 1.072-1.075]; heterogeneity P = .000; random-effects odds ratio, 1.366 [95% CI, 1.303-1.432]) showed an increased risk in relation to preterm delivery, with substantial heterogeneity between study-specific estimates. The effect of preterm delivery on asthma was stronger in cross-sectional studies; studies with broad outcome criteria, a small sample size, and a younger study population; and studies conducted in English-speaking populations, outside Europe, and published more recently. In metaregression, adjusting for other determinants, the effect estimate was significantly associated only with the mean age of the study population.
  
  
  CONCLUSIONS
  The weight of evidence shows that preterm babies have an increased risk of asthma compared with term babies.
  
  
  CONCLUSIONS
  Recognition of prematurity as a determinant of asthma emphasizes the importance of active treatment of physiologic airflow obstruction and a need for special preventive measures against known environmental determinants of asthma in preterm babies.
authors.affiliation: Institute of Occupational and Environmental Medicine, University of Birmingham, UK. j.jaakkola@bham.ac.uk
authors.forename: Jouni J K
authors.initials: JJ
authors.lastname: Jaakkola
authors.forename: Parvez
authors.initials: P
authors.lastname: Ahmed
authors.forename: Antonia
authors.initials: A
authors.lastname: Ieromnimon
authors.forename: Petra
authors.initials: P
authors.lastname: Goepfert
authors.forename: Elpiniki
authors.initials: E
authors.lastname: Laiou
authors.forename: Reginald
authors.initials: R
authors.lastname: Quansah
authors.forename: Maritta S
authors.initials: MS
authors.lastname: Jaakkola
mesh_terms: D001249:Asthma / Q000453:epidemiology* / Q000209:etiology*; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D008297:Male; D011247:Pregnancy; D047928:Premature Birth / Q000503:physiopathology*; D012107:Research Design; D012307:Risk Factors