What are fibroids?

Fibroids are tumours that grow in the uterus (womb). They are benign, which means they are not cancerous, and are made up of muscle fibre. Fibroids can be as small as a pea and can grow as large as a melon. It is estimated that 20-50% of women have, or will have, fibroids at some time in their lives. They are rare in women under the age of 20, most common in women in their 30s and 40s, and tend to shrink after the menopause.Although the exact cause of fibroids is unknown, they seem to be influenced by oestrogen. This would explain why they appear during a woman's middle years (when oestrogen levels are high) and stop growing after the menopause (when oestrogen levels drop).

According to US studies, fibroids occur up to nine times more often in black women than in white women, and tend to appear earlier*. The reason for this is unclear. Also women who weigh over 70kg may be more likely to have fibroids. This is thought to be due to higher levels of oestrogen in heavier women.

In the past, the contraceptive pill was thought to increase the risk of fibroids, but that was when the pill contained higher levels of oestrogen than it does today. Some studies suggest that the newer combined pill (oestrogen and progestogen) and the mini pill (progestogen only) may actually help prevent or slow the growth of fibroids.