Menstruation (mammal)

[11] Overt menstruation (where there is blood flow from the vagina) occurs primarily in humans and close evolutionary relatives such as chimpanzees.

[7] Some species, such as domestic dogs, experience small amounts of vaginal bleeding while approaching heat;[12] this discharge has a different physiologic cause than menstruation.

[21] There are varying views on evolution of overt menstruation in humans and related species, and the evolutionary advantages in losing blood associated with dismantling the uterine lining rather than absorbing it, as most mammals do.

[21] Most female placentals have a uterine lining that builds up when the animal begins ovulation, and later further increases in thickness and blood flow after a fertilized egg has successfully implanted.

For this reason, the human uterine lining becomes fully thickened during each cycle as a defense to trophoblast penetration of the endometrial wall,[23] regardless of whether an egg becomes fertilized or successfully implants in the uterus.

[24] Since most aneuploidy events result in stillbirth or miscarriage, there is an evolutionary advantage to ending the pregnancy early, rather than nurturing a fetus that will later miscarry.

[medical citation needed] The female will ovulate spontaneously and be receptive to the male to be bred (express estrus) at regular biologically defined intervals.

For breeding livestock, there are a number of advantages to be gained by finding methods to induce ovulation on a planned schedule, and thus synchronize the estrus cycle between many female animals.

In order to induce estrus, a variety of techniques have been tried in recent years, involving both more natural, and more hormonal based methods.