Anal sphincter tears: prospective study of obstetric risk factors

Objective To evaluate intrapartum risk factors for anal sphincter tear. Design A prospective observational study. Setting Delivery unit at the University Hospital in Goteborg, Sweden. Participants 2883 consecutive women delivered vaginally during the period b etween 1995 and 1997. Information was obtained from patient records and fro m especially designed protocols which were completed during and after child birth. Main outcome measures Anal sphincter (third and fourth degree) tear. Results Anal sphincter tear occurred in 95 of 2883 women (3.3%). Univariate analysis demonstrated that the risk of anal sphincter tear was increased b y nulliparity, high infant weight, lack of manual perineal protection, defi cient visualisation of perineum, severe perineal oedema, long duration of d elivery and especially protracted second phase and bear down, use of oxytoc in, episiotomy, vacuum extraction and epidural anaesthesia. After analysis with stepwise logistic regression, reported as odds ratio, 95% confidence i nterval, the following factors remained independently associated with anal sphincter tear: slight perineal oedema (0.40, 0.26-0.64); manual perineal p rotection (0.49, 0.28-0.86); short duration of bear down (0.47, 0.24-0.91); no visualisation of perineum (2.77, 1.36-5.63); parity (0.59, 0.40-0.89); and high infant weight (2.02, 1.30-3.16). Analysis of variance showed that manual perineal protection had a stronger influence on lowering the frequen cy, and lack of visualisation of perineum and infant weight had a stronger influence on raising the frequency, of anal sphincter tears in nulliparous compared with parous women. Conclusions Perineal oedema, poor ocular surveillance of perineum, deficien t perineal protection during delivery, protracted final phase of the second stage, parity and high infant weight all constitute independent risk facto rs for anal sphincter tear. Such information is essential in order to reduc e perineal trauma during childbirth.