Background: During childbirth, many women sustain trauma to the perineum, which is the area between the vaginal opening and the anus. These tears can involve the perineal skin, the pelvic floor muscles, the external and internal anal sphincter muscles as well as the rectal mucosa (lining of the bowel). When these tears extend beyond the external anal sphincter they are called 'obstetric anal sphincter injuries' (OASIS). When women sustain an OASIS, they are at increased risk of developing anal incontinence either immediately following birth or later in life. Anal incontinence is associated with significant medical, hygiene and social problems. Endoanal ultrasound (EAUS) can be performed with a bedside scanner by inserting a small probe into the anus and the structures of the anal canal and perineum can be reviewed in real-time. We proposed that by examining the perineum with EAUS after the birth of the baby and before the tear has been repaired, there would be an increase in detection of OASIS. This increased detection could lead to improved primary repair of the external and internal anal sphincter resulting in reduced rates of anal incontinence and improved quality of life for women. EAUS may also have a role after perineal repair in the evaluation of residual injury and may help guide a woman's management in subsequent pregnancies and allow for early referral to specialised units, minimising long-term complications. Objectives: To evaluate the effectiveness of EAUS in the detection of OASIS following vaginal birth and in reducing the risk of anal sphincter complications related to OASIS. Search methods: We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (31 August 2015) and reference list of the one retrieved study. Selection criteria: Randomised control trials (RCTs) comparing EAUS versus no ultrasound in women prior to repair of perineal trauma and EAUS performed after perineal repair. RCTs published in abstract form only and trials using a cluster-randomised design were eligible for inclusion, but none were identified. Trials using a cross-over design and quasi-RCTs were not eligible for inclusion. Data collection and analysis: The two review authors independently assessed the single trial for inclusion and assessed trial quality. Both review authors independently extracted data. Data were checked for accuracy. Main results: We included one trial that randomised 752 primiparous women with clinically detectable second-degree perineal tears to either further assessment with EAUS prior to perineal repair or standard care. We assessed this trial as being at a low risk of bias. The trial reported women's anal incontinence at three and 12 months as well as their pain scores and quality of life assessment. The trial authors reported outcomes at three months for 719 women (364 in the experimental group, 355 in the control group, 4% loss to follow-up), and an outcome at 12 months for 684 women (342 in the experimental group, 342 in the control group, 9% loss to follow-up). Primary outcome Compared with clinical examination (routine care), the use of EAUS prior to perineal repair was associated with a reduction in the rate of severe anal incontinence (defined as involuntary loss of faeces or flatus that constitutes social and/or hygiene problems, or as defined by authors), at greater than six months postpartum (risk ratio (RR) 0.48, 95% confidence interval (CI) 0.24 to 0.97, 684 women at the 12-month time point). Secondary outcomes Severe anal incontinence at less than six months was reduced with the use of EAUS prior to repair when compared with clinical examination (routine care) (RR 0.38, 95% CI 0.20 to 0.72, 719 women). However, increased perineal pain at three months was associated with the use of EAUS prior to perineal repair when compared with routine care (RR 5.86, 95% CI 1.74 to 19.72, 684 women). There was no clear difference in the number of women who reported any anal incontinence at either less than six months or equal to or greater than six months (outcomes not prespecified in our published protocol). Similarly, there was no clear difference between groups in terms of faecal incontinence, flatal incontinence, faecal urgency, or maternal quality of life. The study did not report any data on the need for secondary repair of external anal sphincter, dyspareunia, women's satisfaction with care or the planned or actual mode of birth in any subsequent pregnancy. We were unable to assess the detection rates of OASIS with EAUS from the included study because women with clinically-detected OASIS were excluded from randomisation. Authors' conclusions: There is some evidence to suggest that EAUS prior to perineal repair is associated with reduced risk of severe anal incontinence but an increase in the incidence of perineal pain at three months postpartum. However, these results are based on one small study involving 752 women. The study took place in a large teaching hospital with an average to busy labour ward. The trial participants were similar to those found in most large obstetric units in developed countries, thus increasing applicability of the evidence, but were restricted to primiparous women. More research is needed to further evaluate the effectiveness of EAUS in the detection of OASIS following vaginal birth and in reducing the risk of anal sphincter complications related to OASIS. More high-quality RCTs are needed to fully evaluate the intervention before the routine use of EAUS on the labour ward could be supported. It would be particularly useful if future trials could assess detection rates of OASIS with EAUS versus clinical examination alone as this is the basis of the theory for improved outcomes with this intervention. Cost and the training required to implement EAUS should be considered, along with maternal quality of life and individual symptoms experienced by postnatal women. It would also be useful to follow up women after their subsequent vaginal births to determine if subsequent mode of delivery affects long-term outcomes. Future studies in multiparous women may also be useful.