Risk factors for obstetric anal sphincter injuries (OASIS) and the role of episiotomy: A retrospective series of 496 cases.

OBJECTIVE:The objective of this study was to determine the prevalence and identify risk factors for obstetric anal sphincter injuries (OASIS), and to determine the prevalence of episiotomy and whether it is protective for the posterior perineum. STUDY DESIGN:This is a retrospective case-control study carried out in a level 2 maternity unit and a level 3 maternity unit between 1 January 2006 and 31 December 2015. The sample population included all vaginal deliveries at term of a living singleton foetus in cephalic presentation. The case group comprised patients with an OASIS. The control group comprised patients without OASIS. Statistical analysis was subdivided into descriptive and inferential parts. RESULTS:42,626 patients were included in the study of whom 496 were cases of OASIS, i.e. a rate of 1.2%. The overall episiotomy rate was 10.0%, which reflects a restrictive practice. Episiotomy doesn't appear to be a statistically significant protective factor for OASIS (OR=0.89-95%CI [0.68-1.16]). The principal independent risk factors for OASIS were nulliparity (ORa=4.19-95%CI [3.03-5.84] - p<0.001), previous caesarean (ORa=5.59-95%CI [3.68-8.44] - p<0.001), uterine fundal height greater than 32cm (ORa=1.35-95%CI [1.03-1.77] - p=0.03), gestational or pre-pregnancy diabetes (ORa=1.76-95%CI [1.22-2.46] - p=0.002), birthweight of more than 3500g (ORa=1.48-95%CI [1.17-1.87] - p=0.001), assisted delivery (ORa=1.81-95%CI [1.18-2.86] - p=0.007), and use of a second instrument or obstetrical manoeuvre (ORa=1.93-95%CI [1.05-3.30] - p=0.02). CONCLUSION:Episiotomy doesn't appear to be a statistically significant protective factor on the perineal prognosis. A deeper understanding of the factors which promote OASIS and greater awareness of them would improve the perineal prognosis of parturient women.