The onset of menstruation (the menarche) is determined partly by genetic factors - women start their periods at much the same age as their mothers - but mostly by physical development; typically a young girl will begin to menstruate shortly after her weight passes 41kg (6 1/2 stone). If she is over this weight, aged 16 and has still not started, a full-scale investigation will be needed to determine the cause, which may be a genetic or hormonal disorder. At first, menstruation does not indicate fertility; for her first few cycles a girl is likely not to be ovulating. Nothing about ovulation is certain, however, and girls may become pregnant even before they have had their first period. Ovulation usually begins after a year or two of anovulatory periods, and the change is commonly marked by the onset of dysmenorrhoea.
Once regular menstruation is established, quite a jolt is needed to suppress it. One increasingly common way for this to occur is by loss of weight. A teenage girl who begins to starve herself, either for reasons of fashion or at the start of the serious, poorly understood illness anorexia nervosa, is likely to cease menstruation as her weight returns below the critical level of 40kg or so. The loss of weight may result from dedication to some sporting activity such as long-distance running, but the effect is the same. Amenorrhoea in these circumstances is a warning sign: it shows that ovulation has ceased, hormone production has slowed, and if nothing is done she risks her bones thinning and even fracturing from osteoporosis.
The contraceptive pill stops ovulation and stops menstruation, too. The regular bleeding that occurs while a woman is taking the Pill is withdrawal bleeding that results from the seven days in each 28 that she is not taking the hormones. When a woman stops taking the Pill, it may be several months before she starts to menstruate again. A woman who has not been taking hormones and has not lost weight but whose periods have unexpectedly stopped should not ignore the message from her body. Amenorrhoea may be due to something wrong with the ovaries, underactivity of the pituitary or thyroid glands, or a number of other hormonal disorders. It may also be due to emotional stress or depression; no one really knows how or why the mind is able to exert this effect upon the ovaries.