The cycle is designed so that a woman produces an egg once each month, allowing her the possibility of becoming pregnant.
Each cycle begins with the first day of the period. A number of eggs begin to grow in the ovary until, after fourteen days, one egg is mature enough to be released from the ovary (ovulation).
While developing, the cells around the egg produce the hormone oestrogen. After ovulation these cells produce a second hormone, progesterone, and together the two hormones stimulate the lining of the uterus (womb) to grow. The ovulated egg enters the fallopian tube and travels towards the womb. If intercourse has taken place and no contraception has been used, the sperm may fertilise the egg while it is in the tube and the resulting embryo will implant into the lining of the womb where it develops into a baby. The human race is not very fertile and it has been calculated that a woman who is not using contraception has on average only a 20 per cent chance of conception each cycle.
If the egg is not fertilised it does not implant and fourteen days after ovulation the lining of the womb, together with some blood, is shed into the vagina and the whole process starts all over again. The shedding of the lining of the uterus is the period or ‘menstruation’. Most women have a period once each month. Cycle length (the start of one period to the start of the next) varies among individuals, but anything between 21 and 35 days is considered normal. Of course an individual woman’s cycle will vary from month to month by two or three days, but this is also normal.
Some women, however, have periods which come less often than once every 4-5 weeks, i.e. they bleed infrequently. Gynaecologists call this condition oligomenorrhoea – oligo means few and menorrhoea essentially means having periods. If a woman has not had a period for a long time the term amenorrhoea (no periods) is used. The changes to the endometrium during a woman’s monthly cycle are influenced directly by the hormones that are released by the ovaries as result of the natural maturation of the ovarian follicles. If this function becomes abnormal it will affect the normal woman’s cycle as well.