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The menstrual cycle and hormonal changes

Mr Amit Shah, Mr Anil Gudi & Prof Roy Homburg

Menstruation normally occurs once a month between the time of puberty up to the menopause. Although periods may be irregular for the first year or two after their onset and also nearing the menopause, a normal menstrual cycle should have a duration of 24-35 days. A frequency outside these limits is considered as abnormal.

In the normal menstrual cycle, the central event is ovulation, the release of a mature egg from one of the ovaries which is capable of being fertilized by sperm to produce a pregnancy. Ovulation, which may occur from either ovary with no fixed pattern, is followed about 14 days later by menstruation, assuming conception has not happened. In a regular cycle of 28 days, ovulation will therefore occur around day 14 if the count starts on day 1 of the period. A ‘missed’ period in many cases is the first sign of a pregnancy.

The process of ovulation is the culmination of a wonderfully integrated and synchronized succession of hormonal actions and anatomical changes within the ovary. The hormones that are involved originate mainly in two areas of the brain, the hypothalamus and the pituitary gland and from the ovaries themselves. The major players in this system are gonadotrophin releasing hormone (GnRH) from the hypothalamus whose purpose is to stimulate the release of the gonadotrophins, follicle stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary gland which in turn are carried in the blood to the ovaries. There they have a number of duties including stimulating the ovaries to produce estrogen (FSH), androgen hormones and progesterone (LH) and to select and develop a follicle containing the egg to be ovulated (“egg of the month”). The hormones produced by the ovaries influence changes in the lining of the uterus as if preparing it every month for a possible pregnancy to be taken in. Should this not occur, a drop in the levels of estrogen and progesterone cause the lining of the uterus to disintegrate and bleed (menstruation).

For all these highly sophisticated changes to occur, every part of the system must ‘talk’ to the other parts in a constant series of messages and these feedback mechanisms cause an ebb and flow in the amount and timing of the discharge of all these hormones. Many other factors are involved in the fine tuning of these events which leaves one in awe of the ingenuity of the system and a little surprised that it does not breakdown more often than is actually seen!

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