Menstruation (mammal)
Most female mammals have an estrous cycle, yet only ten primates species, four bats species, the elephant shrew, and the spiny mouse have a menstrual cycle. The relatedness of these groups suggests that four distinct evolutionary events have caused menstruation to arise.
Ovulatory shift hypothesis
Most female mammals experience reproductive fertility cycles. They typically consist of a long period of low fertility, and a brief period of high fertility just prior to and including ovulation. In humans, this is called the ovulatory cycle, or menstrual cycle. The period of high fertility is also called the fertile window, and is the only time during the cycle when sex can result in conception.