Menstrual cycle is regular natural change that occurs in the female reproductive organ that make pregnancy possible.

In short it is monthly changes in the uterus.

The first day of menstrual cycle is marked by onset(beginning) of menstruation - coming out of blood - Flow phase.

The interval of follicle maturation before ovulation is the follicular phase

During this time, cells around the egg secrete estrogens that stimulates the endometrium - wall of the uterus to thicken.

The rise in estrogen encourages the pituitary glands to release the LH hormone. The flow of LH causes the follicles to swell and burst. Then LH triggers - መቀስቀስ ovulation. Then Ovulation occurs.

After ovulation takes place Luteal phase of cycle begins. LH stimulates formation of corpus luteum - secrets some estrogens and a lot of progesterone. As I wrote in 4.7 progesterone and estrogens cause the uterine lining to thicken and encourage blood vessels to grow through it. After The uterus is ready for pregnancy.

A women enters menopause period when all follicles in her ovaries have either been released during menstrual cycle or have disintegrated as the result of aging. If there is no follicle, also there is no production of progesterone and estrogen then the menstrual cycle cease.

This process of menopause is known only in humans and whales(only 2 species).