What process is most likely occurring in the endometrium just before the onset of bleeding

Apoptosis

Explanation:

Apoptosis is also known as "the death of cells", it is a cellular process that occurs when cells undergoes self destruction, this process helps to keep the body healthy and also protects the body. Apoptosis plays an important role by helping the body to get rid of viral infections within the cell before it spreads.

Scientists have discovered that an adult human being go through the process of apoptosis losing about 50-70 billion cells every day while for a child between 8-14 years, 20-30 billion of cells die every day due to apoptosis.

Apoptosis occurs shortly before menstruation via hormonal stimuli, in which there is the breaking down of the endometrium, it sloughs off and then undergo regeneration.