title: Is the risk of obstetric anal sphincter injuries increased in vaginal twin deliveries?
abstract: OBJECTIVE
  Vaginal twin deliveries have a higher rate of intrapartum interventions. We aimed to determine whether these characteristics are associated with an increased rate of obstetric anal sphincter injuries compared with singleton.
  
  
  METHODS
  Retrospective study of all twin pregnancies undergoing vaginal delivery trial was conducted from January 2000-September 2014. Sphincter injury rate compared with all concurrent singleton vaginal deliveries. Multivariable analysis was used to determine twin delivery association with sphincter injuries while adjusting for confounders.
  
  
  RESULTS
  About 717 eligible twin deliveries. Outcome was compared with 33 886 singleton deliveries. Twin pregnancies characterized by a higher rate of nulliparity (54.8% versus 49.5%, p = 0.005), labor induction (42.7% versus 29.1%, p < 0.001), and instrumental deliveries (27.5% versus 16.7%, p < 0.001), lower gestational (34.6 ± 3.3 versus 38.8 ± 2.3, p < 0.001), and lower birth weight. Total breech extraction was performed in 29.0% (208/717) of twin deliveries. Overall obstetric sphincter injury rate was significantly lower in the twins group (2.8% versus 4.4%, p = 0.03, OR = 0.6, 95% CI 0.4-0.9), due to lower rate of 3rd degree tears in twins versus singletons (2.2% versus 4.0%, p = 0.02), rate of 4th degree tears similar among the groups (0.6% versus 0.4%, p = 0.5). In multivariable analysis, sphincter injuries were associated with nulliparity (OR = 3.9, 95% CI 3.4-4.5), forceps (OR = 6.8, 95% CI 5.8-7.8), vacuum (OR = 2.9, 95% CI 2.5-3.3), earlier gestational age (OR = 0.2, 95% CI 0.1-0.3), episiotomy (OR = 0.8, 95% CI 0.7-0.9), and birth weight over 3500 g (OR = 1.8, 95% CI 1.6-2.0). However, the association between twins (versus singletons) deliveries and sphincter injuries was lost after adjustment for delivery gestational age (OR = 0.7, 95% CI 0.4-1.2).
  
  
  CONCLUSIONS
  Despite a higher rate of intrapartum interventions, the rate of sphincter injuries is lower in twins versus singleton deliveries, mainly due to a lower gestational age at delivery.
authors.affiliation: a Division of Feto Maternal Medicine, Department of Obstetrics and Gynecology , Sunnybrook Health Science Centre, University of Toronto , Toronto , Ontario , Canada.
authors.forename: Hadar
authors.initials: H
authors.lastname: Rosen
authors.affiliation: a Division of Feto Maternal Medicine, Department of Obstetrics and Gynecology , Sunnybrook Health Science Centre, University of Toronto , Toronto , Ontario , Canada.
authors.forename: Nir
authors.initials: N
authors.lastname: Melamed
authors.affiliation: a Division of Feto Maternal Medicine, Department of Obstetrics and Gynecology , Sunnybrook Health Science Centre, University of Toronto , Toronto , Ontario , Canada.
authors.forename: Rania
authors.initials: R
authors.lastname: Okby
authors.affiliation: a Division of Feto Maternal Medicine, Department of Obstetrics and Gynecology , Sunnybrook Health Science Centre, University of Toronto , Toronto , Ontario , Canada.
authors.forename: Ori
authors.initials: O
authors.lastname: Nevo
authors.affiliation: a Division of Feto Maternal Medicine, Department of Obstetrics and Gynecology , Sunnybrook Health Science Centre, University of Toronto , Toronto , Ontario , Canada.
authors.forename: Jon
authors.initials: J
authors.lastname: Barrett
mesh_terms: D000328:Adult; D001003:Anal Canal / Q000293:injuries*; D036861:Delivery, Obstetric / Q000009:adverse effects*; D005260:Female; D006801:Humans; D022125:Lacerations / Q000209:etiology*; D015999:Multivariate Analysis; D011247:Pregnancy; D059285:Pregnancy, Twin; D012189:Retrospective Studies
keywords: Anal sphincter injuries; OASIS; instrumental delivery; twin delivery