title: Confounding with familial determinants affects the association between mode of delivery and childhood asthma medication - a national cohort study.
abstract: BACKGROUND
  Mode of delivery may affect the risk of asthma but the findings have not been consistent and factors shared by siblings may confound the associations in previous studies.
  
  
  METHODS
  The association between mode of delivery and dispensed inhaled corticosteroid (ICS) (a marker of asthma) was examined in a register based national cohort (n=199 837). A cohort analysis of all first born children aged 2-5 and 6-9 years was performed. An age-matched sibling-pair analysis was also performed to account for shared genetic and environmental risk factors.
  
  
  RESULTS
  Analyses of first-borns demonstrated that elective caesarean section was associated with an increased risk of dispensed ICS in both 2-5 (adjusted odds ratio (aOR)=1.19, 95% confidence interval (CI) 1.09-1.29) and 6-9 (aOR=1.21, 1.09-1.34) age groups. In the sibling-pair analysis, the increased risk associated with elective caesarean section was confirmed in 2-5 year olds (aOR=1.22, 1.05-1.43) but not in 6-9 year olds (aOR=1.06, 0.78-1.44). Emergency caesarean section and vacuum extraction had some association with dispensed ICS in the analyses of first-borns but these associations were not confirmed in the sibling-pair analyses.
  
  
  CONCLUSIONS
  Confounding by familial factors affects the association between mode of delivery and dispensed ICS. Despite this confounding, there was some evidence that elective caesarean section contributed to a modestly increased risk of dispensed ICS but only up to five years of age.
authors.affiliation: Occupational & Environmental Medicine, Department of Public Health and Clinical Medicine, Umeå University, Umeå, Sweden ; Department of Research and Development, Västernorrland County Council, Sundsvall, Sweden ; Department of Research and Development, Sundsvalls sjukhus, Sundsvall, SE 85186, Sweden.
authors.forename: Lennart
authors.initials: L
authors.lastname: Bråbäck
authors.affiliation: Department of Women's and Children's Health, Division of Reproductive and Perinatal Health, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Cecilia
authors.initials: C
authors.lastname: Ekéus
authors.affiliation: Occupational & Environmental Medicine, Department of Public Health and Clinical Medicine, Umeå University, Umeå, Sweden ; Murdoch Childrens Research Institute, Melbourne, Australia ; Centre for MEGA Epidemiology , School of Population Health, The University of Melbourne, Melbourne, Australia.
authors.forename: Adrian J
authors.initials: AJ
authors.lastname: Lowe
authors.affiliation: Centre for Health Equity Studies (CHESS), Karolinska Institutet/Stockholm University, Stockholm, Sweden ; Clinical Epidemiology, Department of Medicine, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Anders
authors.initials: A
authors.lastname: Hjern
keywords: Asthma; Caesarean section; Child; Epidemiology; Inhaled corticosteroids; Sib pair analysis