The endometrium, which is the innermost layer of the uterus, undergoes changes throughout the menstrual cycle depending on the concentration of estrogen and progesterone at each stage. At the beginning of the menstrual cycle, the sex hormones decrease their concentration and this causes the endometrium to shed with menstruation. The endometrium will then thicken again and change in composition as estrogen and progesterone increase.
After ovulation, in the luteal phase, the endometrium looks trilaminous and becomes receptive. This increases the chances of implantation of the embryo once it reaches the uterus. If pregnancy does not occur, the endometrium is shed again and a new menstrual cycle begins.