Menstruation Is Part of the Uterine Cycle

Menstruation is part of the uterine cycle. Why does it happen?

A) Scientists have not entirely explained why women menstruate.
B) The corpus luteum secretes estrogen and progesterone, causing development of the endometrial ovarian lining in preparation for possible pregnancy. If no pregnancy occurs, hormone levels plummet and the lining is shed.
C) When an egg is released, there is also a release of endometrial fluid from the ruptured follicle. This must be shed in order to prepare the uterus for possible pregnancy.
D) Estrogen levels increase at ovulation, causing the development of the endometrial vaginal lining in preparation for pregnancy. If there is no pregnancy, the lining is shed.
E) When an egg is released, the endometrium or uterine lining develops to support an embryo in case of pregnancy. If pregnancy does not occur, the lining is shed.