Causes
The exact cause of uterine fibroids isn't clear. But these factors may play roles:
- Gene changes. Many fibroids contain changes in genes that differ from those in typical uterine muscle cells.
Hormones. Two hormones called estrogen and progesterone cause the tissue that lines the inside of the uterus to thicken during each menstrual cycle to prepare for pregnancy. These hormones also seem to help fibroids grow.
Fibroids contain more cells that estrogen and progesterone bind to than do typical uterine muscle cells. Fibroids tend to shrink after menopause due to a drop in hormone levels.
- Other growth factors. Substances that help the body maintain tissues, such as insulin-like growth factor, may affect fibroid growth.
- Extracellular matrix (ECM). This material makes cells stick together, like mortar between bricks. ECM is increased in fibroids and makes them fibrous. ECM also stores growth factors and causes biologic changes in the cells themselves.
Doctors believe that uterine fibroids may develop from a stem cell in the smooth muscular tissue of the uterus. A single cell divides over and over. In time it turns into a firm, rubbery mass distinct from nearby tissue.
The growth patterns of uterine fibroids vary. They may grow slowly or fast. Or they might stay the same size. Some fibroids go through growth spurts, and some shrink on their own.
Fibroids that form during pregnancy can shrink or go away after pregnancy, as the uterus goes back to its usual size.
Risk factors
There are few known risk factors for uterine fibroids, other than being a person of reproductive age. These include:
- Race. All people of reproductive age who were born female could develop fibroids. But Black people are more likely to have fibroids than are people of other racial groups. Black people have fibroids at younger ages than do white people. They're also likely to have more or larger fibroids, along with worse symptoms, than do white people.
- Family history. If your mother or sister had fibroids, you're at higher risk of getting them.
- Other factors. Starting your period before the age of 10; obesity; being low on vitamin D; having a diet higher in red meat and lower in green vegetables, fruit and dairy; and drinking alcohol, including beer, seem to raise your risk of getting fibroids.
Complications
Uterine fibroids often aren't dangerous. But they can cause pain, and they may lead to complications. These include a drop in red blood cells called anemia. That condition can cause fatigue from heavy blood loss. If you bleed heavily during your period, your doctor may tell you to take an iron supplement to prevent or help manage anemia. Sometimes, a person with anemia needs to receive blood from a donor, called a transfusion, due to blood loss.
Pregnancy and fibroids
Often, fibroids don't interfere with getting pregnant. But some fibroids — especially the submucosal kind — could cause infertility or pregnancy loss.
Fibroids also may raise the risk of certain pregnancy complications. These include:
- Placental abruption, when the organ that brings oxygen and nutrients to the baby, called the placenta, separates from the inner wall of the uterus.
- Fetal growth restriction, when an unborn baby doesn't grow as well as expected.
- Preterm delivery, when a baby is born too early, before the 37th week of pregnancy.