Pelvis and Perineum Anatomy: Pelvic Floor, Urogenital Triangle, and Clinical Correlations

The pelvis is the bony ring formed by the two hip bones, sacrum, and coccyx. The pelvic floor is a muscular hammock — primarily the levator ani and coccygeus muscles — that closes the pelvic outlet from below and supports the pelvic organs. The perineum is the diamond-shaped region below the pelvic floor, divided by an imaginary line between the ischial tuberosities into the urogenital triangle (anterior, containing the external genitalia) and the anal triangle (posterior, containing the anal canal). The pudendal nerve (S2-S4) is the primary nerve of the perineum and the key to understanding perineal innervation, pudendal nerve blocks, and episiotomy anatomy.

The bony pelvis is divided into the greater (false) pelvis above and the lesser (true) pelvis below by the pelvic brim — an oblique plane running from the sacral promontory posteriorly through the arcuate line of the ilium and the pecten pubis to the pubic symphysis anteriorly. Everything above this line is the greater pelvis (really just the lower abdominal cavity flanked by the iliac blades). Everything below is the true pelvis — the funnel-shaped space that contains the bladder, rectum, and reproductive organs. The true pelvis has three important diameters that matter clinically: the anteroposterior diameter from sacral promontory to pubic symphysis (the obstetric conjugate, normally about 10 cm), the transverse diameter at the widest point (about 13 cm), and the oblique diameter. These measurements matter in obstetrics because the fetal head must navigate through these spaces during delivery. A narrow AP diameter (as in a platypelloid pelvis) increases the risk of obstructed labor. The pelvic outlet is the bottom opening of the true pelvis, bounded by the coccyx posteriorly, the ischial tuberosities laterally, and the pubic arch anteriorly. This is the opening that the pelvic floor muscles close.

The pelvic floor is not a single muscle — it is a muscular sheet made up of the levator ani (which itself has three named parts) and the coccygeus. Together they form the pelvic diaphragm, a hammock that supports the pelvic organs, maintains continence, and must relax during defecation and childbirth. The levator ani has three components, and knowing their names and attachments is high-yield. The pubococcygeus is the largest part — it runs from the pubic bone to the coccyx and to the anococcygeal raphe. The subset of pubococcygeus that loops around the vagina (or prostate in males) is sometimes called the pubovaginalis or puboprostaticus. The puborectalis is the sling of muscle that wraps around the anorectal junction and pulls it forward, creating the anorectal angle — this angle (about 80-90 degrees at rest) is the primary mechanism of fecal continence. When the puborectalis relaxes during defecation, the angle straightens to about 130 degrees and the rectum aligns with the anal canal. The iliococcygeus runs from the tendinous arch of the levator ani (a thickening in the obturator fascia) to the coccyx. The coccygeus (ischiococcygeus) lies posterior to the levator ani, running from the ischial spine to the sacrum and coccyx. It is largely a fibrous sheet at this point in evolution but still contributes to the pelvic floor. Innervation is from the nerve to levator ani (branches of S3, S4) and direct branches of the pudendal nerve for the external sphincter. Pelvic floor weakness — from childbirth trauma, chronic straining, or neurological injury — leads to urinary incontinence, fecal incontinence, and pelvic organ prolapse. This is not an abstract concept. Pelvic floor dysfunction affects roughly one-third of women who have had vaginal deliveries.

The perineum is the shallow space below the pelvic floor. Picture a diamond when looking from below, with the pubic symphysis at the front, the coccyx at the back, and the ischial tuberosities at the sides. Draw a line between the two ischial tuberosities — this divides the diamond into the urogenital triangle (anterior) and the anal triangle (posterior). The urogenital triangle contains the external genitalia and the perineal membrane (the old "urogenital diaphragm" concept has been revised — it is now understood as a single fascial layer called the perineal membrane, not a muscular diaphragm with two layers). In males, the urethra passes through the perineal membrane. In females, both the urethra and the vagina pass through. The superficial perineal space (between the perineal membrane and the superficial perineal fascia) contains the erectile tissues: the bulb of the penis or vestibular bulbs, the crura of the clitoris or penis, and the associated muscles — ischiocavernosus covering the crura and bulbospongiosus covering the bulb. The anal triangle contains the anal canal, the external anal sphincter (voluntary, skeletal muscle, pudendal nerve S2-S4), the internal anal sphincter (involuntary, smooth muscle, autonomic innervation), and the ischiorectal (ischioanal) fossae — fat-filled spaces lateral to the anal canal that accommodate expansion during defecation. These fossae communicate posteriorly behind the anal canal, which explains why perianal abscesses can spread from one side to the other — a horseshoe abscess. The perineal body is a fibromuscular node at the center of the perineum, between the urogenital and anal triangles. Multiple muscles converge here: the bulbospongiosus, external anal sphincter, and superficial transverse perineal muscles. It is the anchor point for perineal support. Damage to the perineal body during childbirth (tears or episiotomy) can compromise continence.

The pudendal nerve (S2, S3, S4 — "S2, 3, 4 keeps the floor off the floor") is the primary nerve of the perineum. It exits the pelvis through the greater sciatic foramen, hooks around the ischial spine and the sacrospinous ligament, re-enters through the lesser sciatic foramen, and runs in a fascial tunnel on the lateral wall of the ischiorectal fossa called the pudendal (Alcock's) canal. It gives off three major branches: the inferior rectal nerve (innervates the external anal sphincter and perianal skin), the perineal nerve (innervates perineal muscles and labial/scrotal skin), and the dorsal nerve of the penis or clitoris (sensory to the glans). Pudendal nerve blocks are performed at the ischial spine — the landmark is palpable through the vaginal wall during labor. The injection targets the nerve as it wraps around the spine. This block anesthetizes the perineum for episiotomy repair and instrumental delivery. Knowing that the pudendal nerve is at the ischial spine is one of the highest-yield clinical facts in pelvic anatomy. AnatomyIQ includes cross-sectional and coronal views of the pelvis that let you trace the pudendal nerve from its sacral roots through the greater sciatic foramen, around the ischial spine, and into the perineum. This content is for educational purposes only and does not constitute medical advice.