title: Impact of ambient air pollution on gestational age is modified by season in Sydney, Australia.
abstract: BACKGROUND
  The effect of individual pollutants and the period(s) during pregnancy when pollutant levels are likely to have most impact on preterm birth is not clear. We evaluated the effect of prenatal exposure to six common urban air pollutants in the Sydney metropolitan area on preterm birth.
  
  
  METHODS
  We obtained information on all births in metropolitan Sydney between January 1, 1998 and December 31, 2000. For each birth, exposure to each air pollutant was estimated for the first trimester, the three months preceding birth, the first month after the estimated date of conception and the month prior to delivery. Gestational age was analysed as a categorical variable in logistic regression models.
  
  
  RESULTS
  There were 123,840 singleton births in Sydney in 1998-2000 and 4.9% were preterm. Preterm birth was significantly associated with maternal age, maternal smoking, male infant, indigenous status and first pregnancy. Air pollutant levels in the month and three months preceding birth had no significant effect on preterm birth after adjusting for maternal and infant covariates. Ozone levels in the first trimester of pregnancy and spring months of conception and sulphur dioxide were associated with increased risks for preterm births. Nitrogen dioxide was associated with a decreased risk of preterm births.
  
  
  CONCLUSIONS
  We found more protective than harmful associations between ambient air pollutants and preterm births with most associations non-significant. In view of these inconsistent associations, it is important to interpret the harmful effects with caution. If our results are confirmed by future studies then it will be imperative to reduce Sydney's already low air pollution levels even further.
authors.affiliation: Centre for Research, Evidence Management and Surveillance, Sydney, Australia. b.jalaludin@unsw.edu.au
authors.forename: Bin
authors.initials: B
authors.lastname: Jalaludin
authors.forename: Trish
authors.initials: T
authors.lastname: Mannes
authors.forename: Geoffrey
authors.initials: G
authors.lastname: Morgan
authors.forename: Doug
authors.initials: D
authors.lastname: Lincoln
authors.forename: Vicky
authors.initials: V
authors.lastname: Sheppeard
authors.forename: Stephen
authors.initials: S
authors.lastname: Corbett
mesh_terms: D000293:Adolescent; D000328:Adult; D000393:Air Pollutants / Q000009:adverse effects* / Q000032:analysis; D005260:Female; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D016015:Logistic Models; D008297:Male; D008423:Maternal Age; D008875:Middle Aged; D009517:New South Wales; D011247:Pregnancy; D047928:Premature Birth / Q000209:etiology*; D012621:Seasons; D012907:Smoking / Q000009:adverse effects; D014505:Urban Population
chemical_list: D000393:Air Pollutants