ObjectiveTo examine he anatomy of the internal and external anal sphincters in the area of midline obstetric lacerations, to gain insight into sphincter damage and repair.MethodsThe length, craniocaudal extent, and overlap of the internal and external and sphincters in the perineal body were measured in 17 cadavers. Further anatomic observations were made in four sets of whole pelvis crosssections taken in he sagittal, coronal, and transverse planes. During the repair of 20 acute fourth-degree lacerations, observations were made to determine the internal sphincter visibility following birth.ResultsThe external and internal anal sphincters overlap by 17.0 mm (standard deviation [SD]6.9), with the internal sphincter lying between the external sphincter and the anal canal. The internal sphincter extends an additional 12.2 mm (SD 5.9) cranial to the proximal extent of the external sphincter, whereas the caudal margin of the internal sphincter lies 3.7 mm (SD 7.2) cranial to the distal margin of the external sphincter. In pregnant women who sustained a fourth-degree laceration, we found that the internal sphincter can be identified as a rubbery white layer adjacent to the anal submucosa lying between the external sphincter and the anal canal.ConclusionThe internal anal sphincter lies between the anal mucosa and the external anal sphincter and extends more than a centimeter above the cranial margin of the external sphincter, a region where it is disrupted when a fourth-degree obstetric laceration has occurred.