Menstruation (mammal)
Menstruation in mammals is the shedding of the uterine lining (endometrium). It occurs on a regular basis in uninseminated sexually reproductive-age females of certain mammal species.
Calendar-based contraceptive methods
Written references to a "safe period" do not appear again for over a thousand years. Scientific advances prompted a number of secular thinkers to advocate periodic abstinence to avoid pregnancy: in the 1840s it was discovered that many animals ovulate during estrus. Because some animals (such as dogs) have a bloody discharge during estrus, it was assumed that menstruation was the corresponding most fertile time for women. This inaccurate theory was popularized by physicians Bischoff, Félix Archimède Pouchet, and Adam Raciborski. In 1854, an English physician named George Drysdale correctly taught his patients that the days near menstruation are the "least" fertile, but this remained the minority view for the remainder of the 19th century.