Questionnaires were returned by 949 (79.2%) of the 1198 women. These women constituted the study population, which has been described in a previous publication.16
At 3 months after delivery incontinence of stool occurred in 3.1% (29/948) of the women, at least once daily in 0.3% (3/948) (). Incontinence of stool was present in 1.8% (2/114) of the women who underwent cesarean section, 2.9% (23/783) of those who gave birth vaginally without recognized anal sphincter damage, and 7.8% (4/51) of those who gave birth vaginally with recognized anal sphincter damage. Among the women who gave birth vaginally, the univariate RR for incontinence of stool in those with anal sphincter damage compared with those without recognized sphincter damage was 2.8 (95% CI 0.8–9.6) (). On univariate analysis macrosomia (birth weight 4000 g or more), episiotomy (median in all cases), anal sphincter tears and prepregnancy urinary incontinence were predictive of incontinence of stool (RR > 1.25), but other factors examined, including instrumental delivery, were not ().
Among the women with forceps-assisted delivery, none of the 51 with an intact anal sphincter had incontinence of stool, as compared with 3 (13.0%) of the 23 with sphincter lacerations (p < 0.03, Fisher's exact test). Even after we excluded the data for the 51 women in whom an anal sphincter tear was recognized, fecal incontinence tended to be commoner among women with a median episiotomy (4.4% [8/183]) than among those who gave birth vaginally with a first- or second-degree laceration (2.3% [9/386]) (univariate RR 1.9, 95% CI 0.7–4.8). The latter had a risk of fecal incontinence similar to that of women with an intact perineum (2.8% [6/214]) (univariate RR 0.8, 95% CI 0.3–2.3). It was not possible to control simultaneously for multiple predictors in the analysis of incontinence of stool owing to the small number of women affected.
Involuntary escape of flatus was reported by 25.5% (242/948) of the women; in 2.6% (25/948) this occurred at least once daily (). The proportion of women affected was similar whether delivery was cesarean (22.8% [26/114]) or vaginal (25.9% [216/834]) (univariate RR 0.9, 95% CI 0.6–1.3). On univariate analysis operative vaginal delivery (compared with spontaneous delivery), anal sphincter tears and urinary incontinence before becoming pregnant were the only factors associated with incontinence of flatus (RR > 1.25) (). Women with incontinence of stool were more likely than those continent of stool to also have involuntary escape of flatus (65.5% [19/29] v. 24.3% [223/918]) (univariate RR 2.7, 95% CI 2.0–3.6). Neither natural hair colour nor the presence of stretch marks predicted incontinence of stool or of flatus (data not shown).
Overall, 252 women had incontinence of either flatus or stool. Multivariate analysis of predictors of postpartum anal incontinence (i.e., incontinence of either flatus or stool) showed a significant independent association with forceps (compared with spontaneous) delivery but not with vacuum extraction, episiotomy or cesarean section (). Maternal age, body mass index, parity and epidural anesthesia were not included in the adjusted model since they were not associated with anal incontinence on univariate analysis. When we replaced episiotomy and type of delivery by degree of perineal injury in the multivariate model, incontinence of flatus or stool was predicted by anal sphincter laceration but not by lesser degrees of perineal trauma ().
Anal sphincter tears were most strongly predicted by lack of previous vaginal birth: 10.7% (50/468) of women having a first vaginal birth had an anal sphincter tear, as compared with 0.3% (1/367) of women with a previous vaginal birth (RR 39.2, 95% CI 5.4–282.5). None of the 47 women with anal sphincter tears at a previous birth had an anal sphincter tear at the current birth. Median episiotomy and vacuum or forceps delivery were highly significant independent predictors of anal sphincter tears at first vaginal delivery (). Since there were significant interactions between median episiotomy and vacuum and forceps delivery, the relative risk of tears with each combination of interventions is presented in . In multivariate analysis the association between macrosomia and anal sphincter damage was not statistically significant (). The results did not change when this analysis was repeated with birth weight in quintiles: the adjusted RR of anal sphincter injury for the heaviest quintile (greater than 3820 g) compared with the lightest (less than 3050 g) was 1.32 (95% CI 0.55–3.18). The apparent increase in anal sphincter injury with duration of the second stage of labour on univariate analysis disappeared after adjustment for the confounding effects of episiotomy and operative delivery (). No association was found on univariate analysis between anal sphincter injury and perineal massage, stretch marks or epidural anesthesia.