Eighteen studies met the inclusion criteria (n=12,237 participants). None of the studies met all six quality criteria to be judged high quality. One study met five criteria, four studies met four criteria and the rest met three or fewer criteria. Most studies did not appear to control for confounding factors.
Vaginal delivery versus caesarean section: Women who had any type of vaginal delivery compared with a caesarean section had an increased risk of developing symptoms of solid, liquid or flatus anal incontinence. Forceps delivery showed an increased risk compared to caesarean section (OR 2.01, 95% CI 1.47 to 2.74; two studies) as did spontaneous vaginal delivery (OR 1.32, 95% CI 1.04 to 1.68; two studies).
Comparison of different methods of vaginal delivery: Instrumental deliveries resulted in more symptoms of anal incontinence compared with spontaneous vaginal delivery (OR 1.47, 95% CI 1.22 to 1.78; six studies). This was statistically significant with forceps delivery but not for ventouse delivery.
Analysis of severe symptoms of anal incontinence (solid and liquid incontinence) found that only instrumental delivery compared with spontaneous vaginal delivery was statistically significant in increase of symptoms of solid and liquid anal incontinence.(OR 1.91, 95% CI 1.00 to 3.67; four studies). The difference between forceps and ventouse deliveries was significant only when symptoms of flatus incontinence were included (OR 1.51, 95% CI 1.07 to 2.13; four studies).
Full details of all results were provided in the paper.