Perineum
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Direct answer
Diamond-shaped and tilted, the perineum is the region below the pelvic diaphragm, bounded by the pubic symphysis in front, the coccyx behind, the ischiopubic rami and ischial tuberosities at the sides and the sacrotuberous ligaments posterolaterally. An imaginary line joining the two ischial tuberosities divides it into the anterior urogenital triangle, containing the external genitalia and the perineal membrane, and the posterior anal triangle, containing the anal canal within its sphincters and the two ischioanal fossae. At the centre of the diamond sits the perineal body, the fibromuscular node into which muscles converge and through which childbirth either passes intact or tears.
What you must remember
- Boundaries: pubic symphysis anteriorly, coccyx posteriorly, ischiopubic rami and ischial tuberosities anterolaterally, sacrotuberal ligaments posterolaterally.
- Dividing line: a transverse line between the ischial tuberosities separates the urogenital triangle in front from the anal triangle behind.
- Roof: the pelvic diaphragm (levator ani and coccygeus) forms the roof; the skin and superficial fascia are the floor.
- Perineal body: the central tendon between the anal canal and the perineal membrane or vagina, receiving the external anal sphincter, superficial and deep transverse perineal muscles, bulbospongiosus and levator ani fibres.
- Urogenital triangle: contains the external genitalia, the perineal membrane with its attached sphincters, and the superficial and deep perineal pouches.
- Anal triangle: contains the anal canal with its sphincter complex and the wedge-shaped ischioanal fossae with their fat and the pudendal canal.
- Blood and nerve supply: internal pudendal vessels and the pudendal nerve (S2 to S4) through Alcock's canal, with the ilioinguinal nerve supplying the anterior labia or scrotum.
- Obstetric grades: a perineal tear involving the anal sphincter (third degree) or anal epithelium as well (fourth degree) is an obstetric anal sphincter injury requiring repair in theatre.
Protecting the perineal body in childbirth
As the presenting distends the vulva, the perineal body is compressed against the pubic arch and thins from its usual 3 to 4 cm to a paper-thin strip. The midwife's controlled delivery, hand on the perineum, aims to let it stretch rather than tear. When an episiotomy is judged necessary, the cut is made mediolateral from the midpoint of the posterior fourchette at about 60 degrees, through posterior vaginal wall and perineal body but away from the midline — deliberately sacrificing muscle length to spare the anal sphincter, exactly the structure a midline incision endangers. Repair is anatomical: vaginal mucosa first, then the perineal body muscles reapproximated, then skin, in theatre under regional block if the sphincter is involved.
The anatomy grades the damage. A second-degree tear involves perineal muscles but spares the sphincter; a third-degree tear includes the external anal sphincter (partial or full thickness); a fourth-degree tear reaches the anal epithelium. Missed sphincter repair means lifelong flatal and faecal incontinence, which is why the classification, not the blood loss, decides who repairs and where.
Where students slip
The perineum is defined as "the skin between vagina and anus" — that colloquial "obstetric perineum" is the perineal body region only; the anatomical perineum is the whole diamond from symphysis to coccyx. The urogenital and pelvic diaphragms are stacked wrongly: the pelvic diaphragm is the upper, deeper roof, the perineal membrane lies below and in front, which is why the ischioanal fossa lies below the levator but behind the perineal membrane. Finally, the blood supply is credited to the external pudendal artery alone; the deep supply is the internal pudendal, whose branches reach the perineum through the pudendal canal.
Frequently asked questions
What are the boundaries of the perineum?
The pubic symphysis in front, the coccyx behind, the ischiopubic rami and ischial tuberosities at the sides, and the sacrotuberous ligaments posterolaterally.
How is the perineum divided and by what line?
Into the urogenital triangle in front and the anal triangle behind, by a transverse line joining the two ischial tuberosities.
What is the perineal body and which muscles converge on it?
A fibromuscular central tendon between anal canal and perineal membrane or vagina, receiving the external anal sphincter, superficial and deep transverse perineal muscles, bulbospongiosus and levator ani slips.
Why is a mediolateral episiotomy preferred to a midline cut?
A midline incision runs straight towards the anal sphincter; a mediolateral cut at about 60 degrees from the fourchette trades a longer, more bleeding cut for sphincter protection.
What is an obstetric anal sphincter injury?
A third-degree tear involving the external anal sphincter with or without the internal sphincter, or a fourth-degree tear breaching the anal epithelium, requiring recognition and repair in theatre.
What supplies the perineum?
The internal pudendal artery and the pudendal nerve (S2 to S4) via the pudendal canal, supplemented anteriorly by the ilioinguinal nerve and external pudendal vessels.