The ovaries are paired organs about the size of walnuts lying one each side of the pelvis. They have two functions. First, they release a ripened egg each month and, second, they produce progesterone and oestrogen — two important female hormones. The ovaries are remarkable organs — when a baby girl is born she already has all the eggs (30,000—40,000) in her ovaries. After puberty the eggs begin to ripen or mature under the influence of complicated hormonal changes that occur cyclically. This usually happens every month (unless the woman is pregnant or on the Pill; nor does it happen for some of the time she is breastfeeding) until the end of her reproductive life, the menopause, intervenes. In practice, several eggs begin to mature each month but for some unknown reason only one usually ripens. The average woman has 400—450 cycles in a lifetime. The ripening and release of an egg each month is called ovulation.
A woman’s body functions in a cyclical way, each cycle lasting about a month. It is important to remember that this is only an average and that there can be large variations in cycle length that are still quite normal. In a classical ‘text-book’ cycle the events run as follows. The first day of the cycle is taken as the first day of menstruation. This is the day when the lining of the womb, now that it is not going to be needed for pregnancy, starts to be shed in the form of clots, cells and blood. The brain (particularly the hypothalamus) influences a tiny gland that lies near it (the pituitary gland) to produce a hormone called follicle-stimulating hormone (FSH) which
stimulates the ovary to ripen an egg that month. Another hormone, luteinizing hormone, (LH) is produced by the pituitary during the whole cycle. A surge of LH is produced around the middle of the cycle (day 14) which helps release the ripened egg. This release of the egg is called ovulation and it is at this stage that a woman is most likely to conceive. Some women experience abdominal discomfort at this time.
Each month, then, an egg is released from one of the ovaries and is ‘collected’ by the tube. It takes at least three days for an egg to pass along the tube. While this is going on, the ruptured egg sac on the ovary turns into a functioning gland called the corpus luteum which produces another hormone called progesterone. Progesterone, together with oestrogen, acts on the lining of the uterus. Oestrogen produced in the first half of the cycle primes the endometrium and encourages growth. Without it, progesterone produced in the second half of the cycle could not act properly to ripen the endometrium ready to receive the fertilised egg. If the egg is not fertilised, the corpus luteum which has a lifespan of only about fourteen days, begins to cease functioning. Progesterone and oestrogen levels then fall, causing the lining of the womb to shed thereby starting a menstrual period.
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