title: Are women having a vaginal birth after a previous caesarean delivery at increased risk of anal sphincter injury?
abstract: OBJECTIVE
  To examine the incidence of obstetric anal sphincter injury in women who had a successful vaginal birth after a previous caesarean delivery (VBAC).
  
  
  METHODS
  Retrospective analysis of prospectively gathered data.
  
  
  METHODS
  A tertiary referral university institution.
  
  
  METHODS
  All secundiparous women with a previous caesarean delivery who had a VBAC from 2001 to 2011.
  
  
  METHODS
  Details of maternal demographics, intrapartum characteristics and outcomes were examined in cases of VBAC with accompanying anal sphincter injury.
  
  
  METHODS
  Rates of obstetric anal sphincter injury and associated risk factors.
  
  
  RESULTS
  During the study period there were 3071 trials of labour in secundiparous women with a previous caesarean delivery; 65% (1981/3071) of these had a successful VBAC. Women having a VBAC were at greater risk of anal sphincter injury than nulliparous women having a vaginal delivery over the same period (5% [98/1981] versus 3.5% [1216/34,496], P = 0.001, odds ratio 1.4, 95% CI 1.15-1.75). The rate of instrumental delivery in woman having a VBAC was 39% (771/1981). On multiple logistic regression analysis an increased rate of instrumental delivery was a strong predictor of sphincter injury (P = 0.03, odds ratio 1.15, 95% CI 1.01-1.3). When the first labours of women with sphincter injury in the VBAC group were examined, 70% (60/86) had been in labour before undergoing their caesarean delivery.
  
  
  CONCLUSIONS
  The incidence of anal sphincter injury in women undergoing VBAC is 5% and birthweight is the strongest predictor of this. The rate of instrumental delivery in this group was also increased.
authors.affiliation: National Maternity Hospital, Dublin, Ireland.
authors.forename: M P
authors.initials: MP
authors.lastname: Hehir
authors.forename: M
authors.initials: M
authors.lastname: Fitzpatrick
authors.forename: M
authors.initials: M
authors.lastname: Cassidy
authors.forename: M
authors.initials: M
authors.lastname: Murphy
authors.forename: C
authors.initials: C
authors.lastname: O'Herlihy
mesh_terms: D000328:Adult; D001003:Anal Canal / Q000293:injuries*; D001724:Birth Weight; D005113:Extraction, Obstetrical / Q000009:adverse effects* / Q000706:statistics & numerical data; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D007231:Infant, Newborn; D016015:Logistic Models; D017063:Outcome Assessment, Health Care; D011247:Pregnancy; D012189:Retrospective Studies; D012307:Risk Factors; D016064:Vaginal Birth after Cesarean / Q000009:adverse effects*
keywords: Anal sphincter injury; morbidity; secundiparous; trial of labour after caesarean delivery; vaginal birth after previous caesarean delivery