How does the menstrual cycle work?

Baby girls are born with all the egg cells in their ovaries that they will have in their lifetime. These egg cells lie in small pockets called follicles. Once puberty starts, different hormones ripen the first follicle so that it will 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 (also called ovarian tube) into the uterus. The moment the egg cell leaves the ovary is called ovulation. Once a girl has had her first monthly period (menstruation), ovulation usually occurs once a month.

 

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 together with the egg cell and some blood is shed through the vagina. This is the monthly period, also called menstruation. As long as a woman is not pregnant and does not use 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.

In order to shed the mucous membrane and the blood, the muscles of the uterus tighten and relax in an irregular rhythm. This allows the tissue to become detached from the wall of the uterus and leave the body through the vagina.

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.

At around 50 years of age women have their last period. Menopause is the medical term for this ending of the menstrual cycles. 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 ripen at all.

Author: German Institute for Quality and Efficiency in Health Care (IQWiG)

© IQWiG (Institute for Quality and Efficiency in Health Care) Last update of this website: 05/21/2012