Fibroid degeneration occurs in pregnancy because the fibroid, which is effectively an overgrowth of muscle in the wall of the uterus, enlarges under the effect of the circulating oestrogens and outstrips its blood supply. This causes thrombosis of the vessels around the outside of the fibroid, and congestion and swelling. The middle of the fibroid becomes red and soft - a process called red degeneration - causing pain and possibly fever, usually during the second part of pregnancy. Since we don't know what causes fibroids we don't know how to stop them, and we can't stop them degenerating during the pregnancy. The only treatment is pain relief and rest.