title: Season and preterm birth in Norway: A cautionary tale.
abstract: BACKGROUND
  Preterm birth is a common, costly and dangerous pregnancy complication. Seasonality of risk would suggest modifiable causes.
  
  
  METHODS
  We examine seasonal effects on preterm birth, using data from the Medical Birth Registry of Norway (2,321,652 births), and show that results based on births are misleading and a fetuses-at-risk approach is essential. In our harmonic-regression Cox proportional hazards model we consider fetal risk of birth between 22 and 37 completed weeks of gestation. We examine effects of both day of year of conception (for early effects) and day of ongoing gestation (for seasonal effects on labour onset) as modifiers of gestational-age-based risk.
  
  
  RESULTS
  Naïve analysis of preterm rates across days of birth shows compelling evidence for seasonality (P < 10(-152)). However, the reconstructed numbers of conceptions also vary with season (P < 10(-307)), confounding results by inducing seasonal variation in the age distribution of the fetal population at risk. When we instead properly treat fetuses as the individuals at risk, restrict analysis to pregnancies with relatively accurate ultrasound-based assessment of gestational age (available since 1998) and adjust for socio-demographic factors and maternal smoking, we find modest effects of both time of year of conception and time of year at risk, with peaks for early preterm near early January and early July.
  
  
  CONCLUSIONS
  Analyses of seasonal effects on preterm birth are demonstrably vulnerable to confounding by seasonality of conception, measurement error in conception dating, and socio-demographic factors. The seasonal variation based on fetuses reveals two peaks for early preterm, coinciding with New Year's Day and the early July beginning of Norway's summer break, and may simply reflect a holiday-related pattern of unintended conception.
authors.affiliation: National Institute of Environmental Health Sciences, Research Triangle Park, NC, USA, weinber2@niehs.nih.gov.
authors.forename: Clarice R
authors.initials: CR
authors.lastname: Weinberg
authors.affiliation: National Institute of Environmental Health Sciences, Research Triangle Park, NC, USA.
authors.forename: Min
authors.initials: M
authors.lastname: Shi
authors.affiliation: Department of Global Public Health and Primary Care, University of Bergen, and Medical Birth Registry of Norway, Bergen, Norway and.
authors.forename: Lisa A
authors.initials: LA
authors.lastname: DeRoo
authors.affiliation: Department of Obstetrics and Gynecology, and Department of Epidemiology, Biostatistics and Occupational Health, McGill University, Montreal, QC, Canada.
authors.forename: Olga
authors.initials: O
authors.lastname: Basso
authors.affiliation: Department of Global Public Health and Primary Care, University of Bergen, and Medical Birth Registry of Norway, Bergen, Norway and.
authors.forename: Rolv
authors.initials: R
authors.lastname: Skjærven
mesh_terms: D005260:Female; D005306:Fertilization; D005865:Gestational Age; D006693:Holidays; D006801:Humans; D009664:Norway / Q000453:epidemiology; D011247:Pregnancy; D047928:Premature Birth / Q000453:epidemiology* / Q000209:etiology*; D016016:Proportional Hazards Models; D012307:Risk Factors; D012621:Seasons
keywords: Preterm birth; harmonic analysis; seasonality