The uterine cycle is the cyclic operation of the uterus whose function is to prepare the lining of the uterus, called the endometrium, to receive a possible embryo in case of fertilization. The beginning of the uterine cycle is marked by the onset of menstruation which generally lasts from 3 to 5 days; this blood loss corresponds to the removal of the surface layer of the endometrium which is rich in blood vessels. Following this removal, the endometrium begins thickening again, stimulated by estrogen hormone secretion. After ovulation, progesterone secretion allows the endometrium to undergo the ultimate changes that will make it suitable to accommodate the embryo if there is fertilization. In the absence of fertilization, the uterine cycle begins again. The average cycle length is 28 days.