Adenomyosis is a non-cancerous condition in which the lining cells of the uterus grow directly into the muscle wall of the uterus. When the lining cells of the uterus bleed during the menstrual period, these misplaced cells in the muscle wall bleed as well, and the blood seepage causes pain.
As blood accumulates in the muscle wall, the surrounding muscle swells and forms fibrous tissue in response to the irritation. This swollen area within the uterine muscle wall — called an adenomyoma — feels very much like a fibroid during a medical examination and is often confused with a fibroid on a sonogram.
Fewer patients are diagnosed with adenomyosis than with fibroids. Patients with adenomyosis usually complain more about the pain than the bleeding while, typically, in patients with fibroids the bleeding is worse than the pain. Of course, the only way to be sure is to get a proper diagnosis is by a doctor who is familiar with fibroids and adenomyosis.