Levator ani, the main muscular sling supporting the pelvic organs
The levator ani is a U-shaped muscular sling that supports the pelvic organs (the uterus, bladder and rectum) from below and stretches around the baby's head in vaginal birth.
Remember
The levator ani is the main muscular sling that holds the pelvic organs up, and it is the muscle group that stretches, and can be injured, in vaginal birth, which is a recognised cause of later prolapse.
Memory aid
Picture the levator ani as a hammock slung under the pelvic organs
Use the picture to recall that the muscle holds the organs up from below and has to stretch to let the baby through.
The levator ani is a U-shaped set of muscles that encircles the urethra, vagina and rectum, and it fans out like a hammock beneath the bladder, uterus and rectum.
It is the largest part of the pelvic floor, the muscular layer across the bottom of the pelvic cavity, and it has three parts: pubococcygeus, puborectalis and iliococcygeus. Each part is named after where it attaches; pubococcygeus, for example, runs from the pubis (the front of the pelvis) to the coccyx.
The levator ani gives the main dynamic support: living muscle that keeps a resting tone (a steady low level of contraction) and can tighten when needed, for example against a cough or lifting. Ligaments around the organs anchor them to the pelvic walls, but they are connective tissue, not contracting muscle, so they work with the sling and do not replace it.
In vaginal birth the head has to get through the opening in the sling, so the muscle stretches around it; in computer models, regions of the levator ani reach roughly 2.3 to 3.3 times their resting length. In some women the overstretching tears the muscle, and one important injury is levator ani avulsion, where the muscle tears away from its attachment. It is detected after roughly 10% to 30% of vaginal deliveries.
The pushing force in labour comes from the contracting uterus and the mother, not from the pelvic floor, which has to give way. Damage to the sling and its attachments leaves the organs without support from below, so they can slide down: this is pelvic organ prolapse, where the bladder, uterus or rectum sags into the vagina. Weakness or tearing can also lead to stress urinary incontinence (leaking urine when abdominal pressure rises) and faecal incontinence (leaking stool).
Key facts
- Parts
- the levator ani is made up of pubococcygeus, puborectalis and iliococcygeus.
- Sling support
- the strong, sling-like support from below that keeps the pelvic organs in the pelvic cavity, which the levator ani provides.
- Dynamic support
- living muscle with a resting tone, so it tightens when abdominal pressure rises, as in coughing or lifting.
- Stretch in birth
- in computer models of vaginal birth, regions of the levator ani stretch to roughly 2.3 to 3.3 times their resting length, and the muscle can tear.
- Greatest stretch
- in computer models, the medial pubococcygeus (the part nearest the midline) stretches the most, so it is at greatest risk of stretch injury.
- Later risk
- overstretching or tearing in vaginal birth can contribute to prolapse and incontinence.
Exam traps
- The coccygeus (a small triangular muscle behind the levator ani) is part of the pelvic floor and does support the pelvic organs, but it is not the main support.
- Obturator internus lies in the lateral wall of the pelvic cavity and rotates the hip, so it does not form the sling that holds the organs up.
- The superficial perineal muscles sit in the perineum (the skin and muscle between the vagina and anus) and are often injured in perineal tears, but they are not the main support, which comes from the deep levator ani.
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