No one knows exactly what causes endometrial cancer, but it occurs when normal cells mutate and multiply. As they accumulate, a tumor begins to form. The abnormal cells can then spread to other parts of the body. There are a number of different types of endometrial cancer, such as adenocarcinoma (the most common), squamous cell carcinoma, carcinosarcoma, small cell carcinoma, undifferentiated carcinoma, clear-cell carcinoma, and transitional carcinoma.

Risk factors for developing endometrial cancer include:

Obesity
Being past menopause
Menstruation that began before age 12
Never having been pregnant
High estrogen levels; changes in the balance of your hormones
Use of hormone replacement therapy with estrogen
Polycystic ovarian syndrome (PCOS)
Diabetes
A personal or family history of breast or ovarian cancer
Hereditary cancer syndromes such as Lynch syndrome (hereditary nonpolyposis colorectal cancer or HNPCC)