How does the menstrual cycle work?

According to present knowledge, ovaries already contain all the egg cells women will have in their lifetime when they are 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 transmitters. They regulate body functions like the menstrual cycle and body temperature. Hormones also trigger ovulation.

What happens during ovulation?

Around the time when a mature egg cell has left the ovary, a woman can get pregnant. The egg cell travels down the fallopian tube into the 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: Uterus


What happens during menstruation?

During the monthly hormone cycle the mucous membranes lining the inside of the uterus prepare for the possibility that an egg might be fertilized and settle in the uterus. The mucous membranes supply nutrients to the embryo if the woman gets pregnant. If the egg cell is not fertilized, it dies.

At the end of the cycle some blood vessels in the mucous membranes of the uterus open up for some time, and the uppermost mucous layer detaches. In order to discharge it, the muscles in the uterus contract and then relax again in an irregular rhythm. That allows the tissue to detach from the wall of the uterus and be shed 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 usually is a sign that one menstrual cycle has finished and the next one has started. In most women, the period lasts 3 to 5 days.

Illustration: Changes of hormone levels and in the lining of the uterus (womb) during the menstrual cycle

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 when they reach menopause, and they completely stop at an average age of 51. A deciding factor for when menopause will begin 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.

Menche N. (ed.) Biologie Anatomie Physiologie. Munich: Urban & Fischer/ Elsevier; 2012.

Pschyrembel W. Klinisches Wörterbuch. Berlin: De Gruyter; 2014.

Schmidt R, Lang F, Heckmann M. Physiologie des Menschen: mit Pathophysiologie. Heidelberg: Springer; 2011.

IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

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Updated on August 10, 2016
Next planned update: 2022


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

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