The rising levels of oestrogen stimulate the growth of the lining of the uterus, known as the endometrium, and signal the pituitary to reduce FSH production; this in turn causes the ovarian egg cells (ovarian follicles) to stop secreting oestrogen. Without the presence of oestrogen the uterine lining breaks down, and blood and endometrial cells are expelled from the uterus into the vagina and then out of the body. After this first menstrual period, which usually takes place between the ages of 11 and 14 years, girls menstruate irregularly to begin with, but after four to six years (between, on average, the ages of 17 and 19 years) the menstrual pattern becomes established. After the first period it may be months or even up to a year before another occurs, but this is nothing to worry about; eventually a regular pattern is established with men- struation occurring usually each month. The menstrual cycle averages 28 days, although it can vary in length from 24 to 34 days. This irregularity does not mean, however, that the girl cannot become pregnant. The menarche should be a cause for celebration, because menstruation is a sign that the hormonal pathways between the pituitary gland, the ovaries and the uterus are functioning correctly. It also tells a girl that she is changing from a girl to a young woman and has entered the stage when she can have children. If menstruation has not occurred by the age of 15 years or so, particularly if the girl in question has a short stature, then a doctor should be consulted. Simple tests will discover whether the girl’s hormone levels are correct. It may just be a case of late development; but if there is some problem, drug therapy may be necessary.

One problem that can follow puberty in girls is a condition known as dysmenorrhoea. It does not usually occur until two years or more after the menarche, and is a cramp-like pain in the lower abdomen that lasts for up to a day from the start of a period. The peak years for this condition, from which about half of all women suffer at some stage in their life, is between the ages of 17 and 25 years.