The hormone that causes changes in the endometrium during the proliferative phase is estrogen. In the presence of estrogen, women experience proliferative changes in the epithelium, glands, mesenchyme and blood vessels on the surface of the endometrium during the proliferative phase, also known as the proliferative phase. This period is located on days 5-14 of the menstrual cycle, and the thickness of the endometrium is 0.5 mm, which can be proliferated to 3-5 mm. The proliferative phase is further divided into early, middle and late phases, as follows: 1. early proliferative phase: days 5-7 of the menstrual cycle, the endometrium is thin, only 1-2 mm; 2. middle proliferative phase: days 8-10 of the menstrual cycle; 3. late proliferative phase: days 11-14 of the menstrual cycle, the endometrium is further thickens further, up to 3-5mm.