Perineal body length as a risk factor for ultrasound-diagnosed anal sphincter tear at first delivery

Introduction and hypothesis: Shortened perineal body (PB) is associated with an increased risk of ultrasound-detected obstetric anal sphincter tear. The objective was to determine if shortened perineal body length (<3 cm) is a risk factor for ultrasound-detected anal sphincter tear at first delivery. Methods: Pregnant nulliparous women were recruited over 18 months. At 35-37 weeks' gestation and 6 weeks' postpartum perineal body length (PB) was measured and subjects completed quality of life questionnaires. Primary outcome was ultrasound-diagnosed anal sphincter tear at 6 weeks post-partum. Secondary outcomes were also assessed. A priori power analysis determined that 70 subjects were needed to detect a difference in anal sphincter tear based on a PB cut-off of 3 cm. Results: Seventy-three subjects completed the study. Mode of delivery was 69.9% spontaneous vaginal, 15.1% operative vaginal, and 15.1% labored cesarean. There were 25 anal sphincter abnormalities (34.2%) seen on ultrasound: 11 (15.1%) internal or external sphincter tears, 3 (4.1%) internal sphincter atrophy, 6 (8.2%) external sphincter thinning, and 7 (9.6%) external sphincter scarring. Only the 11 sphincter tears qualified as abnormal for the primary outcome. In the vaginal delivery group 16.4% (10 out of 61) had a sphincter tear, compared with 8.3% (1 out of 12) in the labored cesarean group (p =0.68). Women with PB<3 had a significantly higher rate of ultrasound-diagnosed anal sphincter tear (40.0% vs 11.1%, p =0.038). When comparing women with and without sphincter tear, there was a significant difference in mean antepartum PB (3.1 vs 3.7 cm, p =0.043). Conclusions: A shortened perineal body length in primiparous women is associated with an increased risk of anal sphincter tear at the time of first delivery.