The myometrium is the muscular wall of the uterus. This layer lies between the endometrium, which is the internal layer of glands lining the uterus, and the outer membranous layer known as the perimetrium. It is made up mainly of smooth muscle, but also contains connective tissue and blood vessels. The main structure of the uterus is formed by the tissue of the myometrium.
During pregnancy, the myometrium stretches and expands to accommodate the growing fetus. It thins out but still provides structure to the uterus. The layer contains arteries that receive blood from larger arteries and then carries it to the placenta, which supports the fetus.
Once a pregnancy is full term and it is time to give birth, the myometrium is key to the labor process. The muscles must contract to successfully push the baby from the uterus. Once the baby has been delivered, the muscles of the myometrium will continue to contract to push out the placenta. This also compresses the blood vessels, which minimizes the loss of blood after delivery.
Sometimes the muscles of the myometrium lose tone, an issue known as uterine atony. This can cause issues during and after the labor process, such as lack of contractions and postpartum hemorrhaging. Atony may be the result of a number of different factors, such as overstretching of the uterus due to multiple fetuses, labor that is too fast or too prolonged, or having given birth more than five times.
One common issue that may affect the myometrium is the appearance of abnormal masses known as uterine fibroids. Also called leiomyoma, these masses are benign, meaning they are not a precursor to cancer, and if they remain small they often do not cause any symptoms or issues. As they grow, however, they can cause problems such as excessive menstrual bleeding, pain and constipation. They can also lead to infertility by blocking the fallopian tubes or preventing implantation of an embryo. Fibroids may need to be removed via surgery, and in some cases a full hysterectomy may be required.
Cancer that starts in the myometrium is known as uterine sarcoma. This type of cancer is fairly rare; the majority of uterine cancers originate in the endometrium. As with many cancers, early detection is key to having a high chance of survival. Tumors may be removed surgically or treated with radiation or chemotherapy.