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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 1st 2008 to Oct 22nd 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.