ABSTRACTObjectiveTo determine the prevalence of obstetric anal sphincter injuries (OASIS) in a cohort of primiparous women and to evaluate their association with demographic, obstetric and ultrasound parameters.MethodsThis was a retrospective analysis of the ultrasound volume datasets of 320 primiparous women, acquired at 5 months postpartum. Tomographic ultrasound imaging (TUI) was used to evaluate the external anal sphincter (EAS). A significant EAS defect was diagnosed if a defect of > 30° was seen in four or more of six TUI slices bracketing the EAS.ResultsSignificant EAS defects were found in 69 women (27.9% of those delivered vaginally). In nine of those a third‐degree tear was diagnosed intrapartum and was sutured. In 60 women with significant defects there was no documentation of sphincter damage at birth, implying unidentified or occult defects (60/69, 87.0%). Among them, 29 had had a second‐degree tear, two a first‐degree tear and three an intact perineum. In 31 cases an episiotomy had been performed, with five extensions to a third‐degree tear. On multivariate analysis only forceps delivery was significantly associated with OASIS.ConclusionsIn this cohort of primiparous women we found OASIS in 27.9% of vaginally parous women, most of which had not been diagnosed in the delivery suite. There seems to be a need for better education of labor‐ward staff in the recognition of OASIS. On the other hand, it is conceivable that some defects may be masked by intact tissue. The significance of such defects remains doubtful. Forceps delivery was the only identifiable risk factor. Copyright © 2013 ISUOG. Published by John Wiley & Sons Ltd.