Adenomyosis and leiomyoma (uterine fibroids) are common, benign causes of heavy, painful periods and an enlarged uterus, but they differ in location and structure. Adenomyosis occurs when endometrial tissue grows into the uterine muscle wall, causing diffuse enlargement, while leiomyomas are distinct, encapsulated muscular tumors. Both can cause severe pelvic pain and infertility. PubMed Central (PMC) (.gov) PubMed Central (PMC) (.gov)