What happens during menstruation?

This video explains why women have monthly bleeding during their menstrual cycle, and how this process is regulated. The menstrual cycle starts with the first day of the period and lasts about 28 days on average. A period usually lasts 3 to 7 days. About 2 ounces of menstrual blood are shed during the monthly period. That’s about 60 milliliters, or about a third of a small plastic cup.

Why do we have periods?

During the monthly cycle the womb prepares for a possible fertilization. The lining of the womb thickens so that a fertilized egg can embed itself there. The lining nourishes the fertilized egg enabling it to continue growing. If the egg is not fertilized and breaks down. The rest of the lining is flushed out of the body during the monthly period. This process is regulated by hormones in the ovaries, as well as by chemical messengers that are made in the brain and in the pituitary gland.

How does the lining of the womb change?

The lining of the womb changes during the monthly cycle in three phases:

  • After the monthly period, the uppermost layer of the lining slowly builds up and the blood supply is increased. This is known as the proliferative phase and happens between day 5 and day 15 of the monthly cycle.
  • The secretory phase lasts from day 16 to day 28. During this phase the lining of the womb has been completely restored and is full of nutrients. It is now ready to receive a fertilized egg and nourish it.
  • If no egg has implanted itself in the lining of the womb, the uppermost layer of the lining is shed in the monthly period between day 1 and day 4 of the new cycle. This is called the menstrual phase. After it is shed, the lining of the womb is flushed out by blood which comes from many small blood vessels.

If a woman has not become pregnant, she gets her period every month. It is only after an egg has been fertilized that the ovaries produce hormones to retain the lining of the womb. An egg that has not been fertilized either travels through the fallopian tube into the womb, or it moves into the abdominal cavity. It dies and is broken down by the cells of the immune system.

The first day of the monthly period indicates that one cycle has ended and a new cycle is beginning. To shed the lining and the blood, the muscles of the womb contract and relax in an irregular rhythm. The tissue becomes detached from the wall of the womb and can leave the body through the vagina. The contractions of the muscles also make sure that the bleeding does not last for too long.

Menstrual problems

If the muscles of the womb cannot contract properly, the bleeding gets heavier. This can happen, if for example there is a non-cancerous growth, such as a myoma in the muscles of the womb, or if a contraceptive coil is placed in the womb.

A lack of the hormone progesterone can also cause periods to become heavier: Without enough progesterone, the growth of the lining of the womb and the small blood vessels are stopped too late. The lining gets too thick, and when it’s shed, bleeding is longer and heavier than usual.

Photos: Thinkstock: www.thinkstockphotos.de
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Updated on May 18, 2022

Next planned update: 2025


Institute for Quality and Efficiency in Health Care (IQWiG, Germany)

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