Influence of the duration of the second stage of labor on the likelihood of obstetric anal sphincter injury.

Abstract

Duration of the second stage of labor has been suggested as an independent risk factor for clinically detectable obstetric anal sphincter injury in low-risk nulliparous women. A retrospective 5-year cohort study was conducted in a UK obstetrics center which included a high-risk delivery unit and a low-risk birthing center. The study included 4,831 nulliparous women with vertex-presenting, single, live-born infants at term, stratified according to spontaneous or instrumental delivery. Binary logistic regression models were used to examine the association between duration of second stage and sphincter injury. Three-hundred twenty-five of 4,831 women (6.7%) sustained sphincter injuries. In spontaneously delivering women, no association between duration of the second stage and the likelihood of sphincter injury was recorded. Factors associated with increased likelihood of sphincter injury included older maternal age, higher birthweight, and Southeast Asian ethnicity. In contrast, for women undergoing instrumental delivery, a longer second stage was associated with an increased sphincter injury risk of 6 percent per 15minutes in the second stage of labor before delivery. For spontaneous vaginal deliveries, duration of the second stage of labor was not an independent risk factor for obstetric anal sphincter injury. The association between prolonged second stage and sphincter injury for instrumental deliveries is likely explained by the risk posed by the use of the instruments themselves or by delay in initiating instrumental assistance. Attempts to modify the duration of the second stage for prevention of sphincter injuries are unlikely to be beneficial and may be detrimental.