title: The association of episiotomy with obstetric anal sphincter injury--a population based matched cohort study.
abstract: OBJECTIVE
  To estimate the independent association of episiotomy with obstetric anal sphincter injuries (OASIS) using first a cross-sectional and then a matched pair analysis.
  
  
  METHODS
  A matched cohort.
  
  
  METHODS
  Data was gathered from the Finnish Medical Birth Register from 2004-2011.
  
  
  METHODS
  All singleton vaginal births (n = 303,758).
  
  
  METHODS
  Women resulting matched pairs (n = 63,925) were matched based on baseline risk of OASIS defined based on parity (first or second/subsequent vaginal births), age, birth weight, mode of delivery, prior caesarean section, and length of active second stage of birth.
  
  
  RESULTS
  In cross-sectional analysis episiotomy was associated with a 12% lower incidence of OASIS (adjusted odds ratio (aOR) 0.88, 95% confidence interval (CI) 0.80 to 0.98) in first vaginal births and with a 132% increased incidence of OASIS in second or subsequent vaginal births (aOR 2.32, 95% CI 1.77 to 3.03). In matched pair analysis episiotomy was associated with a 23% (aOR 0.77, 95% CI 0.69 to 0.86) lower incidence of OASIS in first vaginal births and a 61% (aOR 1.61, 95% CI 1.14 to 2.29) increased incidence of OASIS in second or subsequent vaginal births compared to women who gave birth without an episiotomy. The matched pair analysis showed a 12.5% and a 31.6% reduction in aORs of OASIS associated with episiotomy, respectively.
  
  
  CONCLUSIONS
  A matched pair analysis showed a substantial reduction in the aORs of OASIS with episiotomy, due to confounding by indication. This indicates that results of observational studies evaluating an association between episiotomy and OASIS should be interpreted with caution.
authors.affiliation: Department of Epidemiology, Rollins School of Public Health, Emory University, Atlanta, Georgia, United States of America; Department of Obstetrics and Gynaecology, Kuopio University Hospital, Kuopio, Finland.
authors.forename: Sari
authors.initials: S
authors.lastname: Räisänen
authors.affiliation: Department of Science Service Center, Kuopio University Hospital, Kuopio, Finland.
authors.forename: Tuomas
authors.initials: T
authors.lastname: Selander
authors.affiliation: Department of Epidemiology & Biostatistics, Imperial College London, London, United Kingdom.
authors.forename: Rufus
authors.initials: R
authors.lastname: Cartwright
authors.affiliation: National Institute for Health and Welfare (THL), Helsinki, Finland, Nordic School of Public Health, Gothenburg, Sweden.
authors.forename: Mika
authors.initials: M
authors.lastname: Gissler
authors.affiliation: Department of Epidemiology, Rollins School of Public Health, Emory University, Atlanta, Georgia, United States of America.
authors.forename: Michael R
authors.initials: MR
authors.lastname: Kramer
authors.affiliation: Department of Obstetrics, Oslo University Hospital, Ullevål, Norway, Faculty of Medicine, University of Oslo, Oslo, Norway.
authors.forename: Katariina
authors.initials: K
authors.lastname: Laine
authors.affiliation: Department of Obstetrics and Gynaecology, Kuopio University Hospital, Kuopio, Finland; School of Medicine, University of Eastern Finland, Kuopio, Finland.
authors.forename: Seppo
authors.initials: S
authors.lastname: Heinonen
mesh_terms: D000328:Adult; D001003:Anal Canal / Q000293:injuries*; D001724:Birth Weight / Q000502:physiology; D015331:Cohort Studies; D003430:Cross-Sectional Studies; D004841:Episiotomy / Q000009:adverse effects*; D005260:Female; D006801:Humans; D015994:Incidence; D022125:Lacerations / Q000209:etiology*; D007744:Obstetric Labor Complications / Q000209:etiology*; D036801:Parturition; D011247:Pregnancy; D012306:Risk; D012307:Risk Factors; D055815:Young Adult