Causes

Doctors don't know the cause of uterine fibroids, but research and clinical experience point to these factors:

• Genetic changes. Many fibroids contain changes in genes that differ from those in typical uterine muscle cells.

• Hormones. Estrogen and progesterone, two hormones that stimulate development of the uterine lining during each menstrual cycle in preparation for pregnancy, appear to promote the growth of fibroids.

  Fibroids contain more estrogen and progesterone receptors than typical uterine muscle cells do. Fibroids tend to shrink after menopause due to a decrease in hormone production.

• Other growth factors. Substances that help the body maintain tissues, such as insulin-like growth factor, may affect fibroid growth.
• Extracellular matrix (ECM). ECM is the material that makes cells stick together, like mortar between bricks. ECM is increased in fibroids and makes them fibrous. ECM also stores growth factors and causes biologic changes in the cells themselves.

Doctors believe that uterine fibroids develop from a stem cell in the smooth muscular tissue of the uterus (myometrium). A single cell divides repeatedly, eventually creating a firm, rubbery mass distinct from nearby tissue.

The growth patterns of uterine fibroids vary — they may grow slowly or rapidly, or they may remain the same size. Some fibroids go through growth spurts, and some may shrink on their own.

Many fibroids that have been present during pregnancy shrink or disappear after pregnancy, as the uterus goes back to its usual size.