Stage I endometrial cancer is diagnosed when the cancer remains confined to the uterus and has not spread to other parts of the body. With around 95 percent of people surviving at least five years after diagnosis, this early stage offers an excellent outlook, though understanding the disease, its treatment, and what comes next is essential for anyone facing this diagnosis.

Epidemiology

Endometrial cancer is the most common cancer affecting the reproductive system in women in the United States. Each year, approximately 65,000 women receive a diagnosis, and about 3 percent of women will be diagnosed with uterine cancer at some point during their lives.^[6] The disease mainly develops after menopause, when hormonal changes in the body create conditions that may allow abnormal cell growth in the lining of the uterus.^[6]

Most women with endometrial cancer are diagnosed at an early stage. The National Cancer Institute notes that 66.8 percent of people with endometrial cancer have stage 1 or stage 2 disease at the time of diagnosis.^[1] This is encouraging news, because the earlier the stage at which cancer is found, the better the chances of successful treatment and long-term survival. Stage 1 cancers are considered early cancers and are the easiest to treat.^[4]

Endometrial cancer makes up about 95 percent of all cases of uterine cancer, which is why the terms “endometrial cancer” and “uterine cancer” are often used interchangeably. The remaining small percentage consists of a very rare type called uterine sarcoma, which develops in the muscle wall of the uterus rather than its lining.^[6]

Causes

Researchers are still working to understand the exact cause of endometrial cancer. What they do know is that something triggers changes in the cells of the uterus. These altered cells begin to grow and multiply out of control, which can eventually form a mass called a tumor.^[6] Understanding what causes these cellular changes remains an area of active investigation.

The endometrium is the inner layer of the uterus, and it changes throughout a woman’s menstrual cycle. Hormones called estrogen and progesterone cause the endometrium to thicken each month in preparation for a possible pregnancy. If no pregnancy occurs, the body produces less progesterone, which causes the endometrial lining to shed during menstruation.^[6] This normal hormonal balance can be disrupted by various factors, and many risk factors for endometrial cancer relate to an imbalance between estrogen and progesterone.

Risk Factors

Several factors can increase the chances of developing endometrial cancer. Many of these risk factors relate to the balance between estrogen and progesterone in the body. Having obesity is one significant risk factor, as excess body fat can produce additional estrogen. A condition called polycystic ovarian syndrome (PCOS), which affects hormone levels, also increases risk. Taking unopposed estrogen, meaning taking estrogen without taking progesterone, can raise the likelihood of developing this cancer as well.^[6]

Other health conditions and lifestyle factors may contribute to increased risk. Women with diabetes, those who have never been pregnant, and those who started menstruating early or experienced late menopause may face higher risk. These factors all relate to longer or greater exposure to estrogen without the balancing effect of progesterone. It’s important for women who have multiple risk factors to discuss with their healthcare provider what steps they can take to protect their health.^[6]

Symptoms

The most common symptom of endometrial cancer is irregular vaginal bleeding. For women who have gone through menopause, any vaginal bleeding or spotting, even a slight amount, should prompt a conversation with a healthcare provider. Before menopause, vaginal bleeding between periods can be a warning sign.^[6] Women older than 40 who experience extremely prolonged, heavy, or frequent vaginal bleeding should also seek medical attention.^[6]

Other symptoms may include lower abdominal pain or cramping in the pelvis, just below the belly. Some women notice thin white or clear vaginal discharge if they are postmenopausal.^[6] These symptoms can resemble those of many other conditions, especially other conditions affecting the reproductive organs. That’s why an accurate diagnosis is so important. Any unusual pain or irregular bleeding should be evaluated by a healthcare provider to determine the cause.^[6]

Endometrial cancer is often found at an early stage precisely because it causes symptoms early. The symptom of irregular vaginal bleeding prompts many women to see their doctor, which leads to earlier detection and diagnosis.^[8] This early warning sign is one reason why the outlook for stage 1 endometrial cancer is so favorable compared to cancers that may not cause symptoms until they have spread.

Prevention

While it’s not possible to prevent endometrial cancer entirely, certain steps may help reduce risk. Maintaining a healthy weight through diet and regular physical activity can help, since obesity is a significant risk factor related to excess estrogen production. Women taking estrogen hormone therapy should discuss with their doctor whether adding progesterone is appropriate, as taking estrogen alone can increase endometrial cancer risk.^[6]

Being aware of your body and reporting any unusual bleeding or symptoms promptly can lead to earlier detection, when treatment is most effective. Regular checkups with a healthcare provider offer opportunities to discuss personal risk factors and appropriate screening or monitoring strategies. Women with conditions like PCOS or those with a family history of endometrial or related cancers should have open conversations with their doctors about their individual risk and any preventive measures that might be appropriate.

Pathophysiology

Stage 1 endometrial cancer begins in the endometrium, the inner lining of the uterus, and remains confined to the uterus itself. At this stage, the tumor has not spread to the cervix, lymph nodes, or other parts of the body.^[1] The cancer may be found only in the endometrium, or it may have grown into the myometrium, which is the deeper, muscular layer of the uterus.^[1]

Stage 1 endometrial cancer is divided into three substages based on how aggressive the cancer is and how deeply it has invaded the uterine wall. Stage 1A tumors are the least invasive. For a tumor to be classified as stage 1A, at least one of the following must be true: the tumor is found only in the endometrium, the tumor is nonaggressive and has grown less than halfway into the muscle layer, or the tumor is nonaggressive and affects only the uterus and ovaries.^[1]

Stage 1B tumors are nonaggressive but have grown at least halfway into the muscle layer of the uterus.^[1] The depth of invasion into the myometrium is one of the most important factors in predicting whether the cancer has spread to nearby lymph nodes and in determining the risk of recurrence after treatment.^[12] Doctors use tissue collected during surgery to determine the exact substage, examining cancer cells under a microscope in a process called surgical staging.^[1]

There are several types of endometrial cancer, and specialists called pathologists identify the type by looking closely at cancer cells collected during a biopsy or surgery. The type of cancer and how aggressive it appears under the microscope help determine the exact substage and guide treatment decisions.^[1] Understanding these cellular characteristics helps doctors predict how the cancer might behave and what treatments will be most effective.