No one knows exactly what causes fibroids. However, the growth of fibroids appears to depend on the hormone estrogen. They often grow larger when estrogen levels are high, which occurs during pregnancy. Medications that lower the estrogen level can cause fibroids to shrink.

Signs and symptoms of fibroids

• Heavy uterine bleeding. This is the most common symptom, occurring in 30% of women who have them. The excess bleeding usually happens during the menstrual period. Flow may be heavier, and periods may last longer. Women who have submucous or intramural fibroids are most likely to have heavy uterine bleeding.
• Pelvic pressure and pain. Large fibroids that press on nearby structures such as the bladder and bowel can cause pressure and pain. These tend to cause worse symptoms.
• Infertility. This is a rare symptom. It probably accounts for less than 3% of infertility cases. Fibroids can cause infertility by compressing the uterine cavity. Submucous fibroids can fill the uterine cavity and interfere with implantation of the fertilized egg.
• This is also an unusual symptom of fibroids, probably accounting for only a tiny fraction of the miscarriages that occur.
• Pregnancy complications. Fibroids can greatly increase in size during pregnancy, because of increased levels of estrogen. They can cause pain, and even lead to premature labour.