title: Birth position and obstetric anal sphincter injury: a population-based study of 113 000 spontaneous births.
abstract: BACKGROUND
  The association between birth position and obstetric anal sphincter injury (OASIS) in spontaneous vaginal deliveries is unclear.
  
  
  METHODS
  The study was based on the Stockholm-Gotland Obstetric Database (Sweden) from Jan 1(st) 2008 to Oct 22(nd) 2014 and included 113 279 singleton spontaneous vaginal births with no episiotomy. We studied risk of OASIS with respect to the following birth positions: a) sitting, b) lithotomy, c) lateral, d) standing on knees, e) birth seat, f) supine, g) squatting, h) standing and i) all fours. All analyses were stratified for parity. General linear models were used to calculate risk ratios (RR) adjusted for maternal, pregnancy and fetal characteristics.
  
  
  RESULTS
  The rates of OASIS among nulliparous women, parous women and women undergoing vaginal birth after a caesarean (VBAC) were 5.7%, 1.3% and 10.6%, respectively. The rates varied by birth position: from 3.7 to 7.1% in nulliparous women, 0.6% to 2.6% in parous women and 5.6% to 18.2% in women undergoing VBAC. Regardless of parity, the lowest rates were found among women giving birth in standing position and the highest rates among women birthing in the lithotomy position. Compared with sitting position, the lithotomy position involved an increased risk of OASIS among nulliparous (adjusted RR 1.17, 95% CI 1.06-1.29) and parous women (adjusted RR 1.66, 95% CI 1.35-2.05). Birth seat and squatting position involved an increased risk of OASIS among parous women (adjusted RR [95% CI] 1.36 [1.03-1.80] and 2.16 [1.15-4.07], respectively). Independent risk factors for OASIS were maternal age, head circumference ≥35 cm, birth weight ≥4000 g, length of gestation ≥ 40 weeks, prolonged second stage of labour, non-occiput anterior presentation and oxytocin augmentation.
  
  
  CONCLUSIONS
  Compared with sitting position, lateral position has a slightly protective effect in nulliparous women whilst an increased risk is noted among women in the lithotomy position, irrespective of parity. Squatting and birth seat position involve an increase in risk among parous women.
authors.affiliation: Department of Medicine, Division of Clinical Epidemiology, T2, Karolinska University Hospital, 171 76, Stockholm, Sweden. charlotte.elvander@ki.se.
authors.forename: Charlotte
authors.initials: C
authors.lastname: Elvander
authors.affiliation: Department of Medicine, Division of Clinical Epidemiology, T2, Karolinska University Hospital, 171 76, Stockholm, Sweden. mia.ahlberg@ki.se.
authors.forename: Mia
authors.initials: M
authors.lastname: Ahlberg
authors.affiliation: The Department of Health Sciences, Faculty of Medicine, Lund University, Box 117, 221 00, Lund, Sweden. li.thies-lagergren@med.lu.se.
authors.forename: Li
authors.initials: L
authors.lastname: Thies-Lagergren
authors.affiliation: Department of Medicine, Division of Clinical Epidemiology, T2, Karolinska University Hospital, 171 76, Stockholm, Sweden. sven.cnattingius@ki.se.
authors.forename: Sven
authors.initials: S
authors.lastname: Cnattingius
authors.affiliation: Department of Medicine, Division of Clinical Epidemiology, T2, Karolinska University Hospital, 171 76, Stockholm, Sweden. olof.stephansson@ki.se.
authors.forename: Olof
authors.initials: O
authors.lastname: Stephansson
mesh_terms: D000328:Adult; D001003:Anal Canal / Q000293:injuries*; D001724:Birth Weight; D002508:Cephalometry; D005260:Female; D005865:Gestational Age; D006801:Humans; D007746:Labor Presentation; D022125:Lacerations / Q000453:epidemiology*; D008423:Maternal Age; D007744:Obstetric Labor Complications / Q000453:epidemiology*; D010120:Oxytocics / Q000009:adverse effects; D010121:Oxytocin / Q000009:adverse effects; D010298:Parity; D056888:Patient Positioning / Q000706:statistics & numerical data*; D011247:Pregnancy; D015995:Prevalence; D018570:Risk Assessment; D012307:Risk Factors; D013548:Sweden / Q000453:epidemiology; D016064:Vaginal Birth after Cesarean / Q000706:statistics & numerical data
chemical_list: D010120:Oxytocics; D010121:Oxytocin