A period (menstruation) is normal vaginal bleeding that is a natural part of a healthy monthly cycle for a person with a uterus and ovaries.

Every month, in the years between puberty (typically age 11 to 14) and menopause (typically about age 51), your body readies itself for pregnancy. The lining of your uterus thickens and an egg grows and is released from one of your ovaries.

If pregnancy doesn’t occur, estrogen and progesterone levels fall, eventually hitting a level that tells your body to begin menstruation. During your period, the uterus sheds its lining and it’s passed, along with some blood, out of the body through the vagina.

The average person who menstruates loses about 2 to 3 tablespoons of bloodTrusted Source during their period.

The time between periods (last day to first day) typically averages 28 days, with bleeding typically lasting around 4 to 5 days. However, people can experience longer time between periods, and fewer or more days of bleeding, and still have totally “regular” periods.

As a woman, your period is your body’s way of releasing tissue that it no longer needs. Every month, your body prepares for pregnancy.

The lining of your uterus gets thicker as preparation for nurturing a fertilized egg. An egg is released and is ready to be fertilized and settle in the lining of your uterus.

If the egg is not fertilized, your body no longer needs the thicker lining of the uterus, so it starts to break down and is eventually expelled, along with some blood, from your vagina. This is your period, and once it’s over, the process starts all over again.

Full conversation at :
https://www.healthline.com/health/why-do-women-have-periods#why