After ovulation, the oocyte lives for 24 hours or less without fertilization, while the remains of the dominant follicle in the ovary become a corpus luteum – a body with the primary function of producing large amounts of the hormone progesterone. Under the influence of progesterone, the uterine lining changes to prepare for potential implantation of an embryo to establish a pregnancy. The thickness of the endometrium continues to increase in response to mounting levels of estrogen, which is released by the antral follicle (a mature ovarian follicle) into the blood circulation. Peak levels of estrogen are reached at around day thirteen of the cycle and coincide with ovulation. If implantation does not occur within about two weeks, the corpus luteum degenerates into the corpus albicans, which does not produce hormones, causing a sharp drop in levels of both progesterone and estrogen. This hormone drop causes the uterus to loose its lining in menstruation; it is around this time that the lowest levels of estrogen are reached.