Fibroids are firm, round tumours which develop in the muscular wall of the womb known as the myometrium. They may be as small as a grape or as large as a melon. Fibroids are extremely common - between 50 per cent and 80 per cent of women can be found to have them if you look hard enough. Typically, a woman will have several fibroids in her womb, which are all different sizes.
Fibroid tumours are benign (that is, they don't spread). Although they usually don't cause symptoms, and many women have them without realising, they can cause problems as they get bigger, or because of exactly where they grow in the uterus, or indeed during pregnancy. For example, if fibroids grow so large that they distort the inside of the womb, a woman may find that her periods become very heavy.
However, I must emphasise that most women have few problems with their fibroids and most people conceive and carry pregnancies without any trouble.
Fibroids can cause pregnancy problems
Many people used to worry that fibroids could grow rapidly in pregnancy as the levels of female hormones rose dramatically. Research has shown that most don't actually increase in size, and those that do tend to return to their pre-pregnancy size afterwards.
Even so, there are several ways in which fibroids may affect pregnancy.
Pain: fibroids can cause uncomfortable feelings of pressure or heaviness as they impinge on nearby structures, and may cause sharp pain in the lower back and legs if nerves are compressed. A rare complication of fibroids during pregnancy is known as red degeneration. In this condition there's bleeding into the middle of the fibroid.
This tends to occur in the middle three months of pregnancy and is thought to result from the tumour enlarging rapidly and outgrowing its blood supply. This can be extremely painful but will eventually settle without requiring specific treatment.
Infertility, miscarriage and premature birth: fibroids are thought to account for about two to three per cent of all infertility. If fibroids develop just under the surface lining of the womb this can interfere with the way in which a fertilised egg implants or attaches to the womb.
This may lead to recurrent early miscarriage (usually so early that the woman doesn't even know she's pregnant) and infertility. More rarely a fibroid may block the cervix or the opening of the fallopian tubes into the womb. Later on in pregnancy, fibroids can also disrupt the normal development and expansion of the womb, leading to premature birth.
The most common problem with fibroids in pregnancy is delivering two to three weeks early, which is of little threat to the baby. When a woman with infertility problems is found to have fibroids, the tumours may be removed if they're large, to try to improve the chances of pregnancy. But small fibroids are usually best left alone.
Research suggests that when no other reason for infertility can be found, removing the fibroids increases the chance of conceiving by 40 per cent to 80 per cent.
Obstruction of delivery: very occasionally fibroids grow in the lower part of the womb and cause partial blockage of the birth canal. This may lead to a caesarean operation in order to deliver the baby.
This article was last medically reviewed by Dr Trisha Macnair in August 2008
