A prior cesarean section and incidence of obstetric anal sphincter injury

Obstetric anal sphincter injury (OASIS) following birth may have serious, long-term effects on affected women, including fecal incontinence, despite primary repair. This was a retrospective population-based register study. Women with OASIS grouped by order of vaginal delivery and prior cesarean section (CS) were compared separately with women without OASIS using logistic regression analysis. The aim was to assess an association between prior CS and incidence of OASIS across groups of women categorized according to singleton first, second, and third vaginal deliveries between 1997 and 2007 in Finland. The incidence of OASIS was 1.8 % at a first vaginal delivery after a prior CS compared with 1.0 % at a first vaginal delivery without prior CS. After adjustment prior CS was associated with a 1.42-fold risk of OASIS only at the first vaginal delivery, with no further significant risk after one or two previous vaginal deliveries. One centimeter increase in maternal height was associated with a 2 % decrease in OASIS incidence at the first vaginal delivery. Prior CS is a significant risk factor for OASIS at the first vaginal delivery. This suggests that relative fetopelvic disproportion leading to CS for a first delivery also predisposes to OASIS at a first vaginal delivery since 40 % of the increased incidence of OASIS risk was explained by birthweight and 4 % by maternal height.