In plain words
The internal anal sphincter is a ring of smooth muscle around the anal canal. It works without conscious control and keeps the anus closed when you are at rest. The , a ring of muscle you can squeeze on purpose, sits outside it. During a vaginal birth, a deep tear can reach the sphincter muscles, sometimes including the internal one. Tears that involve the sphincter are called third-degree tears, and tears that go on into the lining of the rectum are fourth-degree. Injury can affect control of gas, stool, or urgency. Many people recover well, and some need repair or a referral to a . Leaking gas or stool, or new urgency, is worth mentioning to a provider at the or sooner.
The deeper version
The internal anal sphincter is a continuation of the circular smooth muscle of the rectum and supplies most of the resting anal pressure, while the external anal sphincter provides voluntary squeeze pressure. involves disruption of the IAS, the EAS, or both, classified as third-degree (3a, 3b, 3c) and fourth-degree (involving rectal mucosa) under RCOG Green-top Guideline 29 (2015). Occult injury, detected on or manometry, can be more common than visible injury in some series. Operative vaginal delivery and a are recognized risk factors (ACOG Practice Bulletin 198, 2018). Fecal incontinence after injury is reported in a substantial minority, with wide variation across series. Controversy persists over routine ultrasound screening and the role of repair technique.
Where it comes up
- Healing a tear or stitchesPostpartum: Your Body
- Perineal massage, tearing, and episiotomyGetting Ready for Birth
- Your pelvic floor after birthPostpartum: Your Body
What mothers here say about internal anal sphincter
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Sources
- American College of Obstetricians and Gynecologists — Practice Bulletin 198: Prevention and Management of Obstetric Lacerations at Vaginal Delivery, 2018
- Royal College of Obstetricians and Gynaecologists — Green-top Guideline 29: The Management of Third and Fourth Degree Perineal Tears, 2015