Risk factors for primary and subsequent anal sphincter lacerations: a comparison of cohorts by parity and prior mode of delivery

Abstract

This study was performed to assess the effect of pregnancy, route of delivery, and parity on the risk of primary and subsequent anal sphincter laceration in women at first vaginal delivery (1st VD), vaginal birth after cesarean delivery (VBAC), or second vaginal delivery (2nd VD). This retrospective cohort study used data from a perinatal database that included all deliveries at Magee-Womens Hospital from 1995 to 2002. Anal sphincter laceration was the primary outcome, defined as third- and fourth-degree perineal lacerations. The adjusted odds ratio (OR) for primary and subsequent anal sphincter laceration at delivery by risk group was estimated using logistic regression models and reported with 95% confidence intervals (CIs). We assessed 20,674 live, singleton, term deliveries at Magee-Womens Hospital from 1995 to 2002, including 13,183 with 1st VD, 6068 with 2nd VD, and 1423 with VBAC. Anal sphincter laceration occurred in 16% of women with 1st VD, 18% with VBAC, and 3% with 2nd VD. Multivariable logistic regression modeling for primary anal sphincter laceration showed that 1st VD had OR of 5.1 and 95% CI 4.4, 5.9, and VBAC had OR of 5.1, 95% CI 4.2, 6.2 when compared with the reference group with 2nd VD. Shown in order for 1st VD, VBAC, and 2nd VD, the following factors, adjusted for the other listed factors, were significantly related to anal sphincter laceration except as noted: forceps, ORs of 3.0, 2.6, 5.5; midline episiotomy, ORs of 2.7, 2.9, 2.9; infant birth weight 3500 g or more, ORs of 1.9, 1.9, 1.1, not significantly different from OR of 1.0; vacuum delivery, ORs of 1.7, 1.8, 1.5, not significantly different from OR of 1.0, and 2nd stage of labor 2 hours or longer, ORs of 1.8, 0.9, 0.9, last 2 not significantly different from OR of 1.0. Of women who had anal sphincter laceration in their first vaginal delivery, 7.2% (76 of 1054 women who had 2 pregnancies) had recurrent laceration in their second vaginal delivery, compared with 2.3% (123 of 5147) of women who had a primary anal sphincter laceration in their second vaginal delivery. Multivariable logistic regression modeling for recurrent anal sphincter laceration yielded the following significant factors: episiotomy, OR 8.5, 95% CI 4.1, 17.7; vertex malpresentation (primarily occiput posterior), OR 4.3, 95% CI 1.4, 12.6; shoulder dystocia, OR 2.7, 95% CI 1.2, 5.8; and infant birth weight 3500 g or greater, OR 1.7, 95% CI 1.1, 2.7. At this institution, women undergoing VBAC are at similarly high risk of anal sphincter laceration, compared with nulliparous women. Women with prior anal sphincter laceration are at 3 times increased risk for subsequent sphincter laceration, compared with women with prior vaginal delivery without sphincter laceration. Pregnancy by itself does not appear to be an important factor in decreasing the risk of anal sphincter laceration in subsequent deliveries.