Background: Women with missed Obstetric anal sphincter injuries (OASIs) are at an increased risk of anal incontinence. Objective: To assess the accuracy of 3D Transperineal Ultrasound (TPUS) compared with clinical examination for detecting OASIs. Design: Prospective Observational longitudinal cohort study. Setting: District General Hospital, UK. Population or sample: Women undergoing their first vaginal delivery immediately postpartum. Methods: Perineal trauma was initially assessed by accouchers and women were then re-examined by a trained research fellow. A 3D TPUS was performed immediately after delivery before suturing to look for OASIs. Main outcome measures: OASIs on clinical examination and on TPUS Main Results: Two hundred and sixty-four women participated and two hundred and twenty-six (86%) delivered vaginally. Twenty-one (9%) sustained OASIs. Six (29%) of these tears were missed by the accoucher but were identified by the trained research fellow. TPUS identified 19 of the 21 (90.5%) OASIs. One percent (n = 2) had sonographic appearances of an anal sphincter defect and were not seen clinically. The positive and negative predictive of TPUS to detect OASIs were 91% and 99% respectively. TPUS identified 91% of OASIs compared to 71% detected by the accoucher. However, this was not statistically significant. Conclusions: More OASIs were identified on TPUS compared to examination. TPUS may have role in improving the detection rate of OASIs. Considering immense training and financial implications of using TPUS, attention needs to be focused on training to accurately identify anal sphincter defects on clinical examination. Funding:none Keywords: transperineal ultrasound imaging, obstetric anal sphincter injury