First, consider the teleological nature of the system, its assumed goal of implanting a fertilized egg. What if a woman has done everything in her power to avoid having an egg implant in her uterus, such as birth control or abstinence from heterosexual sex. Is it still appropriate to speak of the single purpose of her menstrual cycle as dedicated to implantation? From the woman's vantage point, it might capture the sense of events better to say the purpose of the cycle is the production of menstrual flow. Think for a moment how that might change the description in medical texts: "A drop in the formerly high levels of progesterone and estrogen creates the appropriate environment for reducing the excess layers of endometrial tissue. Constriction of capillary blood vessels causes a lower level of oxygen and nutrients and paves the way for a vigorous production of menstrual fluids. As a part of the renewal of the remaining endometrium, the capillaries begin to reopen, contributing some blood and serous fluid to the volume ofendometrial material already beginning to flow." I can see no reason why the menstrual blood itself could not be seen as the desired "product" of the female cycle, except when the woman intends to become pregnant.
//Emily Martin,“Woman in the Body : A Cultural Analysis of Reproduction”, Beacon Press, 1992