Objective: Maternal anal sphincter tears after vaginal delivery are frequently not diagnosed clinically and are associated with subsequent fecal incontinence. This study examined whether diagnosis of these tears by ultrasonography, followed by immediate surgical repair, reduces the occurrence of incontinence. Methods: We conducted a randomized trial involving 752 primiparous women without a clinically evident anal sphincter tear to evaluate the benefit of adding endoanal ultrasonography immediately after vaginal delivery to the standard clinical examination of the perineum. When a sphincter tear was diagnosed, the perineum was surgically explored and the sphincter sutured. The main outcome evaluated was fecal incontinence 3 months postpartum graded by the Wexner incontinence scale, which measures incontinence to flatus and liquid or solid stools, need to wear a pad, and lifestyle alterations. Results: Among women assessed by ultrasonography, 5.6% had a sphincter tear. Severe incontinence was reported 3 months after childbirth by 3.3% of women in the intervention group compared with 8.7% in the control group (risk difference-5.4% ; 95% confidence interval -8.9 to -2.0; P = .002). The benefit of the intervention persisted 1 year after delivery, with 3.2% severe incontinence in the intervention group compared with 6.7% in the control group (risk difference -3.5% ; 95% confidence interval -6.8% to -0.3% ; P =.03). Ultrasonography needs to be performed in 29 women to prevent 1 case of severe fecal incontinence. Conclusion: Ultrasound examination of the perineum after childbirth improves the diagnosis of anal sphincter tears, and their immediate repair decreases the risk of severe fecal incontinence. Objective: Maternal anal sphincter tears after vaginal delivery are frequently not diagnosed clinically and are associated with subsequent fecal incontinence. This study examined whether diagnosis of these tears by ultrasonography, followed by immediate surgical repair, reduces the occurrence of incontinence. Methods: We conducted a randomized trial involving 752 primiparous women without a clinically evident anal sphincter tear to evaluate the benefit of adding endoanal ultrasonography immediately after vaginal delivery to the standard clinical examination of the perineum. When a sphincter tear was diagnosed, the perineum was surgically explored and the sphincter sutured . The main outcome evaluated was fecal incontinence 3 months postpartum graded by the Wexner incontinence scale, which measures incontinence to flatus and liquid or solid stools, need to wear a pad, and lifestyle alterations. Results: Among women assessed by ultrasonography, 5.6% had a sphincter tear. Severe incontinen ce was reported 3 months after childbirth by 3.3% of women in the intervention group compared with 8.7% in the control group (risk difference-5.4%; 95% confidence interval -8.9 to -2.0; P = .002). The benefit of the intervention persisted 1 year after delivery with 3.2% severe incontinence in the intervention group compared with 6.7% in the control group (risk difference -3.5%; 95% confidence interval -6.8% to -0.3%; P = .03). Ultrasonography needs to be performed in 29 women to prevent 1 case of severe fecal incontinence. Conclusion: Ultrasound examination of the perineum after childbirth improves the diagnosis of anal sphincter tears, and their immediate repair decreases the risk of severe fecal incontinence.