title: Risk factors for obstetric anal sphincter injuries in twin deliveries: a retrospective review.
abstract: OBJECTIVE
  Risk factors for obstetric anal sphincter injuries (OASIS) have been well-established in singleton pregnancies. Considering the unique characteristics of twin deliveries, our aim was to identify risk factors for OASIS that are specific to twins.
  
  
  METHODS
  A retrospective study of all vaginal twin deliveries in a tertiary center between 2000 and 2014. Women who experienced OASIS (the OASIS group) were compared with those whose anal sphincter was intact (controls).
  
  
  RESULTS
  Overall 717 women were eligible for the study, of whom 20 (2.8 %) experienced OASIS. Women in the OASIS group were more likely to be nulliparous (95.0 % vs 53.7 %, p < 0.001) and were characterized by a higher gestational age at delivery (36.1 ± 2.5 vs 34.6 ± 3.3, p = 0.04), a higher birth weight for both twin A and twin B (2,507 ± 540 g vs 2,254 ± 525 g, p = 0.03, and 25,49 ± 420 g vs 2,232 ± 606 g, p = 0.004 respectively), and a higher rate of episiotomy (40.0 % vs 14.2 %, p = 0.001), instrumental delivery for twin A (80.0 % vs 13.5 %, p < 0.001) or twin B (80.0 % vs 18.7 %, p < 0.001), and inter-twin delivery interval of over 30 min (20.0 % vs 7.5 %, p = 0.04). The only factor that remained significant on multivariate analysis was instrumental delivery: forceps delivery of twin A (OR = 8.8, 95 % CI 2.6-30.1), vacuum extraction of twin A (OR = 9.2, 95 % CI 2.6-34.6), and forceps delivery of twin B (OR = 15.4, 95 % CI 4.9-48.6). In women with certain combinations of risk factors the risk of OASIS was as high as 30 %.
  
  
  CONCLUSIONS
  The overall rate of OASIS in twins is low and instrumental delivery, especially by forceps, is a risk factor.
authors.affiliation: Maternal-Fetal Medicine Division, Department of Obstetrics and Gynaecology, Sunnybrook Health Sciences Centre, University of Toronto, Ontario, Canada. rosenhadar@gmail.com.
authors.forename: Hadar
authors.initials: H
authors.lastname: Rosen
authors.affiliation: Maternal-Fetal Medicine Division, Department of Obstetrics and Gynaecology, Sunnybrook Health Sciences Centre, University of Toronto, Ontario, Canada.
authors.forename: Jon
authors.initials: J
authors.lastname: Barrett
authors.affiliation: Maternal-Fetal Medicine Division, Department of Obstetrics and Gynaecology, Sunnybrook Health Sciences Centre, University of Toronto, Ontario, Canada.
authors.forename: Rania
authors.initials: R
authors.lastname: Okby
authors.affiliation: Maternal-Fetal Medicine Division, Department of Obstetrics and Gynaecology, Sunnybrook Health Sciences Centre, University of Toronto, Ontario, Canada.
authors.forename: Ori
authors.initials: O
authors.lastname: Nevo
authors.affiliation: Maternal-Fetal Medicine Division, Department of Obstetrics and Gynaecology, Sunnybrook Health Sciences Centre, University of Toronto, Ontario, Canada.
authors.forename: Nir
authors.initials: N
authors.lastname: Melamed
mesh_terms: D000328:Adult; D001003:Anal Canal / Q000293:injuries*; D001724:Birth Weight; D016022:Case-Control Studies; D004841:Episiotomy / Q000706:statistics & numerical data; D005260:Female; D005865:Gestational Age; D006801:Humans; D022125:Lacerations / Q000453:epidemiology*; D010298:Parity; D036801:Parturition; D011247:Pregnancy; D059285:Pregnancy, Twin; D012189:Retrospective Studies; D012307:Risk Factors; D014620:Vacuum Extraction, Obstetrical
keywords: Obstetric anal sphincter injury; Risk factors; Twin vaginal deliveries