Common symptoms include:

• A change in your bowel habits, such as diarrhea, constipation or more-frequent bowel movements
• Dark or red blood in stool
• Mucus in stool
• Narrow stool
• Abdominal pain
• Painful bowel movements
• Iron deficiency anemia
• A feeling that your bowel doesn't empty completely
• Unexplained weight loss
• Weakness or fatigue

Rectal cancer occurs when healthy cells in the rectum develop errors in their DNA. In most cases, the cause of these errors is unknown.

Healthy cells grow and divide in an orderly way to keep your body functioning normally. But when a cell's DNA is damaged and becomes cancerous, cells continue to divide — even when new cells aren't needed. As the cells accumulate, they form a tumor.

With time, the cancer cells can grow to invade and destroy normal tissue nearby. And cancerous cells can travel to other parts of the body.

Inherited gene mutations that increase the risk of colon and rectal cancer

In some families, gene mutations passed from parents to children increase the risk of colorectal cancer. These mutations are involved in only a small percentage of rectal cancers. Some genes linked to rectal cancer increase an individual's risk of developing the disease, but they don't make it inevitable.

Two well-defined genetic colorectal cancer syndromes are:

• Hereditary nonpolyposis colorectal cancer (HNPCC). HNPCC, also called Lynch syndrome, increases the risk of colon cancer and other cancers. People with HNPCC tend to develop colon cancer before age 50.
• Familial adenomatous polyposis (FAP). FAP is a rare disorder that causes you to develop thousands of polyps in the lining of your colon and rectum. People with untreated FAP have a greatly increased risk of developing colon or rectal cancer before age 40.

FAP, HNPCC and other, rarer inherited colorectal cancer syndromes can be detected through genetic testing. If you're concerned about your family's history of colon cancer, talk to your doctor about whether your family history suggests you have a risk of these conditions.

The characteristics and lifestyle factors that increase your risk of rectal cancer are the same as those that increase your risk of colon cancer. They include:

• Older age. The great majority of people diagnosed with colon and rectal cancer are older than 50. Colorectal cancer can occur in younger people, but it occurs much less frequently.
• African-American descent. People of African ancestry born in the United States have a greater risk of colorectal cancer than do people of European ancestry.
• A personal history of colorectal cancer or polyps. If you've already had rectal cancer, colon cancer or adenomatous polyps, you have a greater risk of colorectal cancer in the future.
• Inflammatory bowel disease. Chronic inflammatory diseases of the colon and rectum, such as ulcerative colitis and Crohn's disease, increase your risk of colorectal cancer.
• Inherited syndromes that increase colorectal cancer risk. Genetic syndromes passed through generations of your family can increase your risk of colorectal cancer. These syndromes include FAP and HNPCC.
• Family history of colorectal cancer. You're more likely to develop colorectal cancer if you have a parent, sibling or child with the disease. If more than one family member has colon cancer or rectal cancer, your risk is even greater.
• Dietary factors. Colorectal cancer may be associated with a diet low in vegetables and high in red meat, particularly when the meat is charred or well-done.
• A sedentary lifestyle. If you're inactive, you're more likely to develop colorectal cancer. Getting regular physical activity may reduce your risk of colon cancer.
• Diabetes. People with poorly controlled type 2 diabetes and insulin resistance may have an increased risk of colorectal cancer.
• Obesity. People who are obese have an increased risk of colorectal cancer and an increased risk of dying of colon or rectal cancer when compared with people considered normal weight.
• Smoking. People who smoke may have an increased risk of colon cancer.
• Alcohol. Regularly drinking more than three alcoholic beverages a week may increase your risk of colorectal cancer.
• Radiation therapy for previous cancer. Radiation therapy directed at the abdomen to treat previous cancers may increase the risk of colorectal cancer.