Menstruation.

Menstruation.
Physiology.

Menstruation is the shedding of the functional layer (upper two-thirds) of the endometrium after sex steroid withdrawal.

The role of the functional layer is to prepare the uterus for implantation.

The basal endometrial layer (lower one-third) provides tissue for regeneration during the following cycle.

This shedding process, which consists of three phases (menstrual, proliferative, and secretory), is repeated approximately 300–400 times during a woman's reproductive life.

It also occurs in primates and bats.

Menstruation indicates that conception has not occurred.

OHOAG: source.
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