Desmoid tumors are rare growths that start in connective tissue, the tissue that supports muscles, ligaments, bones, and other body structures. The exact reason someone develops a desmoid tumor is not always known.

There are several potential causes and risk factors that can contribute to the development of a desmoid tumor.

In this article, we’ll discuss six known causes and risk factors for desmoid tumors. You can use this information to ask your doctor questions about your risk. Note that just because you have certain risk factors, it doesn’t guarantee you’ll develop a desmoid tumor.

In most cases, desmoid tumors occur sporadically, meaning they’re not inherited. Sporadic tumors are caused by somatic variants (mutations), which are gene changes that happen during a person’s lifetime and aren’t passed from parents to children.

The most common gene involved in sporadic desmoid tumors is CTNNB1. Around 90 percent of sporadic desmoid tumors can be attributed to variants in CTNNB1. The CTNNB1 gene gives instructions for making the protein beta-catenin, which helps control cell growth and division.

When CTNNB1 has a variant, beta-catenin may become too stable and build up in cells. This buildup can allow cells to grow and divide in an uncontrolled way, leading to a desmoid tumor.

About 10 percent of desmoid tumors are related to a genetic condition called familial adenomatous polyposis (FAP). FAP is an inherited condition caused by a variant in the APC gene.

The APC gene helps regulate beta-catenin. When the APC protein doesn’t work correctly, beta-catenin can build up and contribute to uncontrolled cell growth.

FAP causes hundreds of growths called polyps to form in your intestines. In the early stages of the disease, you may not experience any symptoms. As FAP gets worse, you may experience bloody stool, pain in your abdomen, diarrhea, or constipation.

If you have FAP, your risk of developing a desmoid tumor is about 800 times the general population’s risk.

People with FAP tend to develop intra-abdominal desmoid tumors, which grow in the tissue around the intestines.

If you have a family history of FAP, colorectal cancer, or many colon polyps, tell your doctor.

Depending on various health factors, your care team may recommend genetic counseling, colon evaluation, or other tests.

Research from the Desmoid Tumor Research Foundation shows that females are twice as likely to have desmoid tumors compared to males.

This pattern, along with pregnancy-related cases, has led researchers to study whether hormones may influence desmoid tumors. Current research doesn’t prove that estrogen directly causes desmoid tumors or that all tumors grow because of estrogen.

Antiestrogen medications have been used to treat desmoid tumors based on research showing that drugs like tamoxifen block desmoid tumor cells from using estrogen. However, based on newer research, this approach is no longer in favor.

Pregnancy is another factor associated with desmoid tumors, especially abdominal wall tumors. According to some research, 8 percent to 18 percent of desmoid tumor cases are related to a pregnancy.

Desmoid tumors related to pregnancy typically develop in the abdominal wall muscles during the third trimester of pregnancy or shortly after giving birth.

The reasons for the connection between pregnancy and desmoid tumor development remain unclear. One potential explanation is the abdominal wall stretching needed to make room for a growing fetus. This injury and trauma to the muscles could possibly be behind a desmoid tumor.

There is also the potential impact of estrogen, as mentioned above. If you have desmoid tumors and are thinking of conceiving, talk to your doctor about your risks.

If you have a desmoid tumor, you may still be able to get pregnant. Your doctor will first need to confirm that your tumors aren’t in a location that could harm the fetus. They’ll also continue to monitor your tumors regularly if you become pregnant.

Desmoid tumors can form in places where you previously had an injury or surgery, though this doesn’t mean every injury or surgery can cause a desmoid tumor.

When you’re recovering from trauma, like surgery or a severe injury, your body triggers the wound-healing process. Specialized cells known as fibroblasts create scar tissue to close the wound. Once your body finishes healing, the fibroblasts turn off and go into a resting state.

In some people, the fibroblasts don’t turn off after healing a wound. They stay active and continue making scar tissue, which builds up and eventually forms desmoid tumors. This tends to happen in people with mutations in the CTNNB1 gene.

Examples of injuries or surgeries associated with desmoid tumors include:

• Hernia repair (surgery that repairs weaknesses in the abdominal wall)
• Skin cancer removal
• Blunt-force trauma, such as an injury from a car accident
• Bone realignment surgery to correct alignment of or repair bones and joints

Although not all injuries or surgeries are preventable, careful monitoring after surgery is important if you’re at higher risk for desmoid tumors.

Desmoid tumors can occur at any age, but the majority of diagnoses occur when people are in their 30s and 40s.

Being outside this age range doesn’t rule out a desmoid tumor. If you notice a concerning mass, pain, movement problems, or symptoms that don’t go away, talk with your doctor. They can decide whether imaging, a biopsy, genetic testing, or a referral to a specialist is needed.