No one is certain what causes uterine fibroids, but changing estrogen levels seem to play a part in their growth. When estrogen levels are high, as a result of pregnancy or contraceptive pills, for example, the rate of fibroid growth increases.
About 20 per cent of women of childbearing age have uterine fibroids but they are not usually found in women under 30 years old. They rarely appear in young women before they begin menstruating.
As a woman approaches menopause and her estrogen levels decrease, uterine fibroids are likely to shrink or almost disappear. Apart from being a woman of reproductive age, no other risk factors for fibroids have been found.