Fibroids are most common in women aged 30-40 years, but they can occur at any age. Fibroids occur more often in African American women than in white women. They also seem to occur at a younger age and grow more quickly in African American women.
It is not clear what causes fibroids. Some research suggests that they develop from misplaced cells present in the body before birth. The female hormones estrogen and progesterone appear to be involved in their growth. Levels of these hormones can increase or decrease throughout a woman’s life. For instance, menopause causes a decrease in estrogen. Fibroids often shrink when a woman enters menopause. Hormonal drugs that contain estrogen, such as birth control pills, may cause fibroids to grow.