Illustration showing the location of the ovaries as part of the female reproductive system

What are ovaries?

The two ovaries are the main female reproductive organs. Shaped like an almond, each ovary is about 3 cm long, 1.5 cm wide and 1 cm thick. They lie in the pelvis, one on either side of the uterus. The ovaries have two functions – the development and release of eggs, and the production of hormones. All the eggs (ova) a woman will ever have – and considerably more than she will ever need – are contained in her ovaries when she is born. At birth, there are about two million immature eggs in each ovary. By puberty these are reduced to about 300,000, and only about 400 will be released during the childbearing years. The number of ova in the ovaries steadily decreases during middle life, and at by the time menopause starts only 25,000 are left. The ovum (egg) is the largest single cell in the body, but still needs a powerful microscope to be seen.
Each egg (ovum) is surrounded by a small sac called a follicle. When puberty is reached, a cycle is established in which a few of the egg cells develop each month, with one reaching full maturity. When this happens the follicle bursts and releases the egg in the process called ovulation. A woman is fertile and can become pregnant a day or two either side of ovulation – and not at other times.

When an egg is released, it is swept into the adjacent Fallopian tube, the other end of which connects with the uterus.

What are the functions of the ovaries?

Illustration showing a magnified cut section of an ovaryThe ovaries also produce the hormones oestrogen and progesterone. Oestrogen predominates during the ripening of the egg, which takes about two weeks. It is this hormone that causes the lining of the uterus to thicken and the body to prepare for pregnancy. When the egg is released, the production of the second hormone, progesterone, increases, preparing the lining (endometrium) still further and bringing it to total readiness for a fertilised egg. If there is no conception, the oestrogen and progesterone levels fall suddenly and the uterine lining is shed during menstruation. The whole process then begins again. The monthly cycle continues throughout a woman’s childbearing years from puberty to the menopause.

It is the female hormones that also give a woman her secondary sexual characteristics, for instance her broader hips than the male, her breasts, pubic and armpit hair, and her rounder, more feminine shape.


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