title: Perinatal characteristics, older siblings, and risk of ankylosing spondylitis: a case-control study based on national registers.
abstract: BACKGROUND
  The effect of circumstances and exposures early in life on the risk of developing ankylosing spondylitis (AS) is largely unknown. The purpose of this study was to determine whether perinatal characteristics predict development of AS.
  
  
  METHODS
  AS cases (n = 1960; 59 % men) were defined as listed with a diagnosis of AS at least once in the Swedish National Patient Register and registered in the Swedish Medical Birth Register (born ≥1973). Population controls were retrieved from the Swedish Population Register (n = 8378; mean 4.3 controls/case), matched on birth year, sex and county. Odds ratios (OR) for developing AS were determined through conditional logistic regression, with regard to: birth weight, birth order, season of birth, maternal age, gestational length, size for gestational age, type of birth, mode of delivery, congenital malformations, mothers' country of birth, mothers' civil status and size of delivery unit.
  
  
  RESULTS
  In the univariate analyses statistically significant increases in risk for developing AS were observed for having older siblings (OR 1.18; 95 % Cl 1.06-1.30). No association was observed for the remainder of analysed exposures, although there was a weak association with birth weight below 3000 g (OR 1.19; 95 % CI 1.04-1.37), though not for "low birth weight" <2500 g (OR 0.90; 95 % CI 0.70-1.16). The increase in risk associated with having older siblings was consistent in a multivariate analysis adjusting for possible confounders (OR 1.23; 95 % Cl 1.09-1.39). The direction and magnitude of the point estimates were also consistent in several sensitivity analyses and when stratifying by sex.
  
  
  CONCLUSIONS
  Having older siblings was associated with an increased risk for developing AS. These results need to be repeated and confirmed in other cohorts.
authors.affiliation: Department of Rheumatology and Inflammation Research, Institute of Medicine, Sahlgrenska Academy, University of Gothenburg, Guldhedsgatan 10A, 405 30, Gothenburg, Sweden. ulf.lindstrom@gu.se.
authors.forename: Ulf
authors.initials: U
authors.lastname: Lindström
authors.affiliation: Institution of Public Health and Clinical Medicine/Rheumatology, Umeå University, Lasarettsgatan 7, 901 87, Umeå, Sweden. helena.forsblad.delia@umu.se.
authors.forename: Helena
authors.initials: H
authors.lastname: Forsblad-d'Elia
authors.affiliation: Rheumatology Unit and Clinical Epidemiology Unit, Department of Medicine Solna, Karolinska Institutet, Nobels väg 5, Solna, 171 76, Stockholm, Sweden. johan.askling@ki.se.
authors.forename: Johan
authors.initials: J
authors.lastname: Askling
authors.affiliation: The Parker Institute, Department of Rheumatology, Frederiksberg and Bispebjerg Hospital, Nordre Fasanvej 57, 2000, Frederiksberg, Denmark. larserik_kristensen@yahoo.com.
authors.forename: Lars Erik
authors.initials: LE
authors.lastname: Kristensen
authors.affiliation: Department of Rheumatology and Inflammation Research, Institute of Medicine, Sahlgrenska Academy, University of Gothenburg, Guldhedsgatan 10A, 405 30, Gothenburg, Sweden. elisabeth_lie@yahoo.no.|Department of Rheumatology, Diakonhjemmet Hospital, Diakonveien 14, Vinderen, 0319, Oslo, Norway. elisabeth_lie@yahoo.no.
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Lie
authors.affiliation: Section of Rheumatology, Department of Clinical Sciences, Lund University, Barngatan 2B, 221 85, Lund, Sweden. sofiaexar@hotmail.com.
authors.forename: Sofia
authors.initials: S
authors.lastname: Exarchou
authors.affiliation: Department of Rheumatology and Inflammation Research, Institute of Medicine, Sahlgrenska Academy, University of Gothenburg, Guldhedsgatan 10A, 405 30, Gothenburg, Sweden. lennart.jacobsson@gu.se.
authors.forename: Lennart
authors.initials: L
authors.lastname: Jacobsson
mesh_terms: D000328:Adult; D001724:Birth Weight / Q000502:physiology*; D016022:Case-Control Studies; D005260:Female; D006801:Humans; D008297:Male; D018743:Perinatal Care / Q000639:trends; D011247:Pregnancy; D012042:Registries; D012307:Risk Factors; D012621:Seasons; D035781:Siblings; D013167:Spondylitis, Ankylosing / Q000175:diagnosis* / Q000453:epidemiology*; D013548:Sweden / Q000453:epidemiology; D055815:Young Adult