The results are less clear for anal incontinence. Meta-analyses are difficult to conduct because of large differences between studies with regard to prevalence of anal incontinence and data collection methodology (44).
The short-term effects of mode of delivery in the first year after birth were investigated in a meta-analysis (45), which found a significant odds ratio for anal incontinence of 1.3 (95 % CI 1.04 – 1.7) for spontaneous delivery compared to caesarean section. A single research study supports this conclusion (29). However, several other single research studies found no differences between the two groups in the postpartum period (33, 46, 47).
With regard to permanent consequences, there are no convincing differences between the two modes of delivery. In a cohort study of almost 4,000 women, no significant difference was found in the prevalence of faecal incontinence based on mode of delivery either six or twelve years after the birth (23, 48). The conclusion is the same in several other studies (13, 22, 24, 37, 49). There are studies that indicate that caesarean section may protect against faecal or anal incontinence beyond the postpartum period, but they are in a minority (20) and frequently have methodological defects such as low statistical power, which meant that the findings were not significant (25, 50, 51).
A key question with regard to anal incontinence is whether this occurs primarily in the subgroup with anal sphincter rupture. In a Norwegian study of women with anal sphincter rupture postpartum, the prevalence of anal incontinence was 38 % (52). A crucial problem both in research terms and clinically is that diagnosis and recording of anal sphincter rupture can vary in quality.
A systematic review article concluded that third or fourth grade perineal tear (i.e. anal sphincter rupture) was the only aetiological factor in delivery that was clearly associated with anal incontinence (53). In a study investigating faecal incontinence, the prevalence was the same in the caesarean section group and in the group that had delivered vaginally without clinically diagnosed anal sphincter rupture (8 % in both groups) (33). However, the prevalence was significantly higher in a group of women with anal sphincter rupture following vaginal delivery (17 %). Other studies also indicate that vaginal delivery in itself is not a risk factor for anal incontinence if the anal sphincter is undamaged (54) – (56).
Obstetric factors may be significant for the correlation between mode of delivery and anal incontinence. It is considered that episiotomy should not be performed routinely (54, 57) – (60). Operative vaginal delivery using forceps or vacuum extraction increases the risk of anal sphincter rupture (54, 61) – (64) and thereby the risk of anal incontinence (54, 65). A Norwegian study shows that anal sphincter injury during the first delivery provides an odds ratio of 4.3 (95 % CI 3.8 – 4.8) for repeated anal sphincter injury at the next delivery (66).