As women age, the ovaries stop producing as much of the hormones oestrogen, progesterone and testosterone. An egg is no longer released every month and periods start to change in their nature and frequency. Periods usually start to become less frequent and can take months or years to stop completely.
Menopause can sometimes happen when women are younger as part of a natural process. Sometimes periods stop suddenly, when, for example, the ovaries are removed during surgery. Chemotherapy, and/or radiotherapy to the pelvic area, can affect the ovaries and can cause periods to stop temporarily or permanently. Sometimes early menopause is caused by a genetic condition. Sometimes the cause is unknown.
The decline in oestrogen usually occurs between the ages of 45 and 55.