Fibroids are benign tumours that grow inside the uterus, or within the uterine wall. More than 99% of fibroids are benign, and these benign tumours cannot become cancerous. In addition, having fibroids doesn’t increase the risk that a woman will develop uterine cancer.
Estimates of the number of women affected by fibroids vary: some are as low as 30% of women aged 40 to 60 years old. Other estimates say that up to 70% to 80% of women may have fibroids by the time they reach 50 years of age. However, because only around one-third of fibroids ever grow large enough to cause symptoms, or be detected by a doctor, most women are not diagnosed.
Exactly what causes fibroids to develop is unknown. However, it is thought that fibroids develop from abnormal muscle cells, and that the presence of oestrogen enables the rapid growth of a single abnormal cell into a fibroid tumour.