How hormonal contraceptives like the pill work

Pregnancy is impossible if there is no egg (ovum) that could be fertilised by a man's sperm. Baby girls are born with many unripened eggs in their ovaries. But a woman cannot get pregnant unless one of these eggs has ripened and left the ovary. That process is called ovulation, and once a woman starts having periods (menstruation), it generally happens every month.

During ovulation, a ripened egg is released from the ovary, and travels down the fallopian tubes to the uterus (womb). As soon as a woman has ovulated, it could be possible for her to get pregnant.

Hormones are produced in the body, and they act like chemical messengers. They regulate body functions like the monthly menstrual cycle and body temperature. Hormones trigger and control ovulation too. The hormones in contraceptives are designed to stop a woman ovulating.

For hormonal contraceptives to work and prevent ovulation throughout every single month, the woman needs to use the contraceptives regularly. Even a few days missing a low-dose pill could mean the woman will ovulate. The patch and ring have to be used regularly as well. You can read more about the patch and ring here.

Using any of these medications can also affect the body in other ways than just stopping ovulation. For example, the oral contraceptive pill also thickens the mucus or slippery fluid that is released into the vagina. Periods (menstruation) can become less heavy and menstrual symptoms like pain and bloating might be reduced as well. The hormones can also cause some adverse effects, but serious adverse effects from hormonal contraceptives are not common. It is important, however, not to smoke, because smoking increases the risk of adverse effects.

What happens during ovulation

When a baby girl is born, all of the eggs that she will ever have in her life are already inside her ovaries. These eggs are enclosed in small sacs called follicles. During puberty, hormones activate the first of these follicles to ripen and release an egg. The release of an egg is known as ovulation. This is the beginning of the fertile phase of a woman's monthly cycle. From this time onwards the hormones make a new follicle ripen each month, and the follicle produces hormones itself.





During the monthly hormonal cycle the membranes lining the uterus (known as the endometrium) change in preparation for a fertilised egg to implant itself. If the egg is not fertilised, it dies. Then, at the end of the cycle, it is shed from the body through the vagina, together with the membranes. This is what a period (menstruation) is. If a woman is not pregnant and is not using hormonal contraceptives, her period is usually a sign that her last cycle has finished and a new one has started.


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: 07/22/2010