What are fibroids? Fibroids are abnormal growths that develop in or on a woman’s uterus. Sometimes these tumors become quite large and cause severe abdominal pain and heavy periods. In other cases, they cause no signs or symptoms at all. The growths are typically benign, or noncancerous. The cause of fibroids is unknown. Fibroids are also known by the following names: leiomyomas myomas uterine myomas fibromas Fibroids are non-cancerous growths that develop in or around the womb (uterus). The growths are made up of muscle and fibrous tissue, and vary in size. They're sometimes known as uterine myomas or leiomyomas. Many women are unaware they have fibroids because they do not have any symptoms.