title: Obstetric anal sphincter injuries in vaginal delivery of twins: associated risk factors and comparison with singletons.
abstract: OBJECTIVE
  Risk factors related to obstetric anal sphincter injuries (OASIS) are known for singleton deliveries. No study to date has described the rate and risk factors involved in twin deliveries.
  
  
  METHODS
  We conducted a retrospective cohort study (1985-2010) of all vaginal twin and singleton deliveries in a single tertiary center. The adjusted odds ratio (OR) for OASIS at delivery was estimated using the logistic regression model.
  
  
  RESULTS
  The study comprised 1,538 and 91,312 patients with vaginal twin and singleton delivery, respectively. Twenty twin (1.27 %) and 2,331 (2.55 %) singleton deliveries were complicated with OASIS. The following OASIS-associated risk factors were shared by both populations: nulliparity [twins adjusted OR 5.9, 95 % confidence interval (CI) 1.7-20.9; singletons adjusted OR 3.9, 95 % CI 3.5-4.4), occipitoposterior (OP) position (twins adjusted OR 3.00, 95 % CI 1.1-8.0; singletons adjusted OR 1.6, 95 % CI 1.3-2.00), birthweight (for each 100 g) (twin adjusted OR 1.1, 95 % CI 1.0-1.2; singletons adjusted OR 1.07, 95 % CI 1.06-1.08), and instrumental delivery (twins adjusted OR 4.3, 95 % CI 1.2-15.4, singletons adjusted OR 2.4, 95 % CI 2.2-2.6).
  
  
  CONCLUSIONS
  Risk factors of nulliparity, OP position, large fetal size, and instrumental delivery were shared by both twin and singleton deliveries. These data will be useful in counselling women carrying twins who intend to deliver vaginally.
authors.affiliation: Department of Obstetrics and Gynecology, Mount Sinai Hospital, University of Toronto, 600 University Ave, Toronto, ON, M5G 1X5, Canada. shay.porat@gmail.com
authors.forename: Shay
authors.initials: S
authors.lastname: Porat
authors.forename: David
authors.initials: D
authors.lastname: Baud
authors.forename: Dan
authors.initials: D
authors.lastname: Farine
mesh_terms: D000328:Adult; D001003:Anal Canal / Q000293:injuries*; D036861:Delivery, Obstetric / Q000009:adverse effects* / Q000706:statistics & numerical data; D005260:Female; D006801:Humans; D009864:Ontario / Q000453:epidemiology; D036801:Parturition; D011247:Pregnancy; D059285:Pregnancy, Twin; D012189:Retrospective Studies; D012307:Risk Factors; D017695:Soft Tissue Injuries / Q000453:epidemiology