What does it mean to have an enlarged endometrium?

Endometrial thickening is generally caused by relatively high levels of estrogen during the follicular phase. A woman's menstrual cycle consists of three stages. The follicular phase refers to the period when the endometrium is "laying down the lining," meaning the endometrial thickness increases, which is induced by estrogen secreted by the ovaries. The second stage is the ovulation phase. In the third stage, the luteal phase, levels of estrogen and progesterone continue to rise, causing the endometrium to become thinner and softer.