There is no known cause for fibroid formation. The female hormone oestrogen, stimulates their growth. Fibroids do stop growing and shrink after the menopause, but they also increase in size during pregnancy while the production of the oestrogen is very high. Fibroids are more likely to develop if you have a family history of this condition.
Fibroids are named depending upon their position in the uterus. Most of the fibroids grow only within the muscle (myometrium) of the uterus (Intramural fibroids). They increase the overall blood flow to the uterus. Some can grow inwards into the uterine cavity (Submucosal fibroids). These are the least common (5% of all fibroids) and they are the type most likely to cause fertility problems. Finally the fibroids that grow outwards from the uterus protruding are called Subserosal fibroids.