Endometrial hyperplasia is characterized by a spectrum of exaggerated proliferative features in the endometrium. It is the result of prolonged estrogenic stimulation unopposed by progesterone. Microscopically, simple hyperplasia shows an increased number of glands that are often cystic, and where stroma is abundant. Glands then show progressive branching, budding and crowding, and the stroma is squeezed out, creating a more complex pattern. In atypical hyperplasia, there is an additional finding of cytological atypia characterized by nuclear enlargement, prominent nucleoli, dyspolarity and a cribriform pattern.
Spontaneous regression of endometrial hyperplasia often occurs after menopause or when the cause of the unopposed estrogen is treated. The hyperplasia may evolve into adenocarcinoma, which occurs at a frequency of 25% with atypical hyperplasia, though rarely with simple hyperplasia.