How does the menstrual cycle work?

It is thought that all of the egg cells a woman will ever have are already inside her ovaries when she is born. These egg cells lie in small pockets called follicles. Once puberty starts, different cause the first follicle to mature and release an egg cell (ovum).

Hormones are substances produced by the body that act as chemical messengers. They regulate body functions like the menstrual cycle and body temperature. Hormones also trigger ovulation.

What happens during ovulation?

Women can get pregnant around the time a mature egg cell has left their ovary. The egg cell travels down the fallopian tube into the womb (uterus). The moment the egg cell leaves the ovary is called ovulation. Once a girl has had her first monthly period, ovulation usually occurs once a month.

Illustration: Female sex organs

Female sex organs

What happens during a menstrual period?

During the monthly hormone cycle, the mucous membranes lining the inside of the womb prepare for a possible pregnancy. The lining becomes thicker so that a fertilized egg can settle in the womb and be supplied with nutrients for a baby to grow. If the egg cell is not fertilized, it dies.

At the end of the cycle, some blood vessels in the mucous membranes of the womb open up for some time, and the uppermost mucous layer detaches. In order to shed this layer, the muscles of the womb tighten and relax in an irregular rhythm. That allows the tissue to break away from the wall of the womb and leave the body through the vagina, together with some blood. This is the monthly period, also called menstruation. As long as a woman is not pregnant and is not using hormonal contraceptives, the period is usually a sign that one menstrual cycle has finished and the next one has started. A period usually lasts between three to five days in most women.

Illustration: Changes in the lining of the womb during the menstrual cycle - as described in the article

Even though menstrual fluid may look like a lot when it is on a pad or a tampon, normally only about 20 to 60 ml of blood are shed during the monthly period: that is only about 4 to 12 teaspoons.

Most women begin to have irregular periods as they approach menopause. Their periods then stop completely at the age of 51 on average. A woman is said to have reached menopause when she hasn't had a period for one year. The time leading up to the last period is known as the menopause transition or perimenopause. A likely deciding factor for when menopause will occur is the number of follicles remaining in the ovaries. Until around the age of 40, the number of follicles drops slowly. After that, the number drops quite quickly, until no more follicles mature.

Brandes R, Lang F, Schmidt R (Ed). Physiologie des Menschen: mit Pathophysiologie. Berlin: Springer; 2019.

Menche N. (ed.) Biologie Anatomie Physiologie. Munich: Urban & Fischer; 2016.

Pschyrembel W. Klinisches Wörterbuch. Berlin: De Gruyter; 2017.

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Updated on August 21, 2019
Next planned update: 2022

Authors/Publishers:

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

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