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Fibroids are muscular tumors that grow in the wall of the uterus (womb). Another medical term for fibroids is "leiomyoma" (leye-oh-meye-OH-muh) or just "myoma". Fibroids are almost always benign (not cancerous). Fibroids can grow as a single tumor, or there can be many of them in the uterus. They can be as small as an apple seed or as big as a grapefruit. In unusual cases they can become very large.
No one knows for sure what causes fibroids. Researchers think that more than one factor could play a role. These factors could be: