Menstrual flow occurs due to lack of which hormone?

After ovulation, the egg only lives for 24 hours or less without fertilization, while the remains of the graffian follicle in the ovary become a corpus luteum; which has a primary function of producing large amounts of progesterone. Under the influence of progesterone, the endometrium (uterine lining) changes to prepare for the potential implantation of an embryo to establish a pregnancy. If implantation does not occur, the corpus luteum will involute, causing sharp drops in levels of both progesterone and estrogen. The hormone drop causes the uterus to shed its lining and egg in a process termed menstruation. So, the correct answer is Progesterone.