More information about the ovaries

The ovaries - one either side of the uterus - lie in a fold of tissue called the broad ligament, made up of peritoneum, the membrane that lines the abdomen. The surface of the ovary is not covered by peritoneum - if it were then the eggs would be trapped inside - rather the surface is bare, so that at ovulation an egg is released into the abdominal cavity, where (usually) it is swept up into the fallopian tube. Each ovary is almond-shaped and about three to four centimetres long, and each contains about 1-2 million cells called germ cells. These have been formed during the development of the woman as a foetus, starting at about three months' gestation. At six months there are roughly 7 million germ cells, but the number declines before birth and continues to decline throughout the woman's life until the menopause, when the ovaries cease to function.

After puberty (which usually occurs when girls reach a certain body mass) hormones secreted by the hypothalamus stimulate the pituitary (see brain in the head section) to produce other hormones which act on the germ cells, wakening them up, as it were, from the state of suspended animation that they have been slumbering in since birth. One of the germ cells will develop, to produce a fluid filled space called a follicle, containing the egg, or ovum. At ovulation the follicle bursts and the egg is caught by the fallopian tube. Sometimes there is a little bleeding from the surface of the ovary, which irritates the peritoneum and causes pain - which is why some women suffer pain in the middle of the menstrual cycle (called Mittel-Schmerz).

After the egg has been released, the cells of the follicle continue to secrete hormones - oestrogen and progesterone - which act on the lining of the uterus to prepare it for the implantation of an embryo should fertilisation occur. The cells of this follicle appear yellow, and so it is called corpus luteum (meaning yellow body). Should fertilisation not occur, the corpus luteum shrinks, hormonal support for the lining of the uterus is withdrawn and it is shed - the process of menstruation. The first day of menstruation is termed day one of the menstrual cycle. In a normal cycle menstrual loss takes five to seven days and is heaviest at the start of the period. The amount of blood lost is very variable and depends, among other things, on the size of the uterus - so that if uterine size is increased by fibroids, for example, so is blood loss. Ovulation usually occurs around about day 12 of the cycle, and menstruation recurs after 28 days. Not every woman however has a 28 day cycle, and not all cycles are regular, particularly at either end of the reproductive life: they may be very irregular during adolescence, and particularly unpredictable during the time leading up to the menopause, which is the day the periods stop. Approaching the menopause the ovaries become increasingly unresponsive to the hormones secreted by the pituitary (which is why blood tests can show whether a woman is around about the menopause: hormone levels get higher and higher as the pituitary tries to drive the ovary to ovulate). Eventually the cells fail to secrete oestrogen and progesterone, causing a reduction in skin elasticity, hair growth, breast density, vaginal secretion, bone density and, in some women, loss of libido and mood swings.