Menstruation
a sanguineous flow from the lining membrane of the uterus, regularly returning once a month.
It generally begins about the 15th year, indicating pubescence, and terminates about the 45th. It is sometimes prolonged, but cases are rare in which women above 50 years have borne children. There is, as a rule, no menstrual flow during pregnancy and lactation. The diseases incidental to the woman as a result of menstruation are numerous, the chief being menorrhagia and dysmenorrhea.
A similar flow of blood from the lining membrane of the uterus of oviparous animals. It generally recurs once a year, usually in the spring, though in the case of some animals from two to six times.
In those that have undergone a change by domestication, as dogs and cats, the recurrence is usually irregular, depending upon various circumstances, as diet, temperature, etc.