Coloured X-ray showing a calcified fibroid, a benign tumour of the uterus (womb). It is visible as a round area near the centre of the pelvic canal. Fibroids consist of smooth muscle bundles and connective tissue that grow slowly within the uterine wall. They may be as small as a pea or as large as a grapefruit. The cause is unknown but is thought to be related to an abnormal response to oestrogen hormones. Large fibroids may exert pressure on the bladder causing discomfort, or on the bowel causing backache or constipation. Symptomless fibroids usually require no treatment whereas large fibroids may be treated surgically.