The endometrium is the inner lining of the uterus. During the ovulation period, this wall thickens as it prepares the uterus for the eventual fertilization of an egg cell. It is composed of a basal layer and a functional layer, in which the latter sloughs off/sheds off during menstruation.
Therefore, it is the ENDOMETRIUM layer of the uterus undergoes dramatic changes in thickness and structure during the uterine cycle responding to hormones.