Causes of Periods, absent

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There are many possible causes. They include:

Excessive exercise. It is particularly common in athletes involved in endurance events.
Being severely underweight, as a result of an eating disorder or illness. Regular menstruation is unlikely if the body mass index (weight in Kg divided by the height in metres squared) is less than 19 (the normal range is 20 to 25).

Work out your body mass index number here
Severe stress or emotional disturbance
A disorder of the ovary that prevents eggs from being produced
Various medical drugs
A reaction to stopping the contraceptive pill. About one woman in 100 will have amenorrhoea for three to six months after stopping use of the pill. This is commonest in women who have taken the pill continuously without a regular pill-free interval.
Polycystic ovary syndrome: a condition associated with multiple cysts in the ovaries, which features menstrual disturbances or absence of menstruation, sterility, obesity and a male distribution of body hair from hormonal disturbances. Severe long-term illness, or under- or over-activity of the thyroid gland.
Abnormal operation of the hormones controlling ovulation and menstruation produced by the hypothalamus gland (which in turn controls the pituitary gland).

Rare causes of amenorrhoea include:

There are rare cases where the true sex of a child is confused because of poorly developed genital organs at birth.