Even though fibroid tumors are common, no one is really sure how or why they develop, or why fibroids occur again and again in one woman and not at all in another.

Although the cause of fibroids is unknown, many physicians believe that these tumors grow when the body responds abnormally to the female hormone estrogen. Others disagree. What definitely is known is that fibroids:

• Do not occur before puberty, when the body does not release estrogen

• Grow rapidly during pregnancy, when the body produces extra estrogen

• Shrink and even disappear as menopause approaches and the body graduallystops making estrogen

• Rarely appear after menopause

• Grow when women take estrogen supplements to make up for the lack of the hormone

Still, there are many who doubt that estrogen is solely responsible for fibroid tumors. Although fibroids develop in some women who have high levels of estrogen, laboratory tests performed for many women with these tumors show completely normal estrogen levels.

In addition, while existing fibroids tend to grow larger when a woman is pregnant, there is little scientific evidence to show that these tumors multiply during pregnancy. Some physicians believe that the increased blood supply that occurs during pregnancy may be the cause of enlargement, since fibroids require a lot of blood to survive.

Doctors used to believe that fibroids would grow bigger if women used oral contraceptives. It's true that birth control pills may contain high levels of estrogen, but now it appears that these pills do not make fibroids grow and may even discourage them. In addition, taking birth control pills may help reduce the heavy bleeding that is a problem for some women.