Endo is the common shorthand for endometriosis, a condition where tissue similar to the lining of the uterus grows in places it shouldn’t, most often in the pelvic area. It affects roughly 10% of reproductive-age women worldwide, about 190 million people, and takes an average of 5 to 11 years to be formally diagnosed.
What Happens in Your Body
Normally, the tissue lining your uterus thickens, breaks down, and sheds each month during your period. With endometriosis, tissue that behaves the same way grows outside the uterus. The most common locations are the pelvic lining (peritoneum), the ovaries, and the tissue between the rectum and vagina.
This tissue still responds to your hormones each cycle. It thickens and bleeds just like the lining inside your uterus, but because it has no way to exit your body, it triggers inflammation, scarring, and adhesions where organs can stick together. Over time, this process can distort pelvic anatomy and create chronic pain.
The leading explanation for how this happens is called retrograde menstruation: during your period, some menstrual tissue flows backward through the fallopian tubes into the pelvic cavity instead of leaving the body. Most people experience some degree of this backflow, but not everyone develops endometriosis. The tissue also needs to evade the immune system, attach to pelvic surfaces, establish its own blood supply, and continue growing. When all of those steps line up, endometriosis takes hold.
Common Symptoms
Pelvic pain is the hallmark symptom, and it goes well beyond normal menstrual cramping. Normal cramps should be tolerable and shouldn’t force you to miss work, school, or daily activities. Endometriosis pain often starts before your period and extends after it ends, and can show up at other times in your cycle too.
Other common symptoms include:
- Lower back or abdominal pain that may be constant or cyclical
- Pain during sex
- Pain during bowel movements or urination
- Fatigue, bloating, constipation, or nausea, especially around your period
- Difficulty getting pregnant
The severity of symptoms doesn’t always match the extent of the disease. Someone with a small amount of tissue growth can have debilitating pain, while someone with widespread growth may have very few symptoms.
Why It Takes So Long to Diagnose
Endometriosis is notoriously slow to diagnose. Studies consistently report delays of 5 to 12 years between when symptoms start and when someone receives a confirmed diagnosis. More recent U.S. data suggests that gap may be shrinking to around 4.4 years, but it remains significant. The delay stems partly from symptoms overlapping with other conditions and partly from the normalization of severe period pain.
Diagnosis typically starts with your clinical history, a pelvic exam, and imaging like transvaginal ultrasound or MRI. These tools can identify larger growths like ovarian cysts caused by endometriosis, but they struggle to detect minimal or mild disease. For those early-stage cases, laparoscopy (a minimally invasive surgery where a small camera is inserted through a tiny incision) remains the gold standard. The true definitive diagnosis comes from examining a tissue sample under a microscope.
Stages of Endometriosis
Endometriosis is classified into four stages using a scoring system developed by the American Society for Reproductive Medicine. Surgeons assign points based on the size and location of tissue growths and the severity of any adhesions, then add them up:
- Stage I (Minimal): 1 to 5 points, small isolated implants with no significant adhesions
- Stage II (Mild): 6 to 15 points, more implants but still relatively superficial
- Stage III (Moderate): 16 to 40 points, deeper implants, possible ovarian cysts, some adhesions
- Stage IV (Severe): over 40 points, extensive deep implants, large cysts, dense adhesions
These stages describe the physical extent of the disease, not how much pain you feel. Someone at Stage I can be in more pain than someone at Stage IV. The staging system is most useful for surgical planning and fertility decisions.
How Endometriosis Affects Fertility
Endometriosis shows up in 30 to 50% of women being evaluated for infertility. It reduces the ability to conceive through several overlapping mechanisms. Adhesions and scar tissue can physically block or distort the fallopian tubes and ovaries. Chronic inflammation creates a hostile environment in the pelvis that can damage eggs and interfere with fertilization. The disease can also reduce ovarian reserve (the number of viable eggs remaining), lower egg quality, and make the uterine lining less receptive to a fertilized embryo implanting.
Having endometriosis doesn’t mean you can’t get pregnant. Many people with the condition conceive naturally or with assistance, but the timeline and path may look different.
Treatment Options
There is no cure for endometriosis, but treatments focus on managing pain and slowing tissue growth. The approach depends on the severity of your symptoms, whether you’re trying to conceive, and how the disease responds over time.
Hormonal Treatments
Because endometriosis tissue is driven by estrogen, most medical treatments work by altering your hormonal environment. Options include combined birth control pills, progestin-only preparations, and medications that suppress estrogen production more aggressively. These treatments can reduce or stop periods, which often eases symptoms. They don’t eliminate existing tissue but can slow its growth and reduce inflammation. Hormonal treatments aren’t an option if you’re actively trying to conceive, since they work partly by preventing ovulation.
Surgery
Surgery aims to remove or destroy endometriosis tissue directly. There are two main approaches. Excision cuts out the diseased tissue along with a margin of healthy tissue, while ablation burns or vaporizes it on the surface. Excision generally produces better long-term results for several reasons: it allows surgeons to see how deep the tissue penetrates, removes the growth more completely, and leaves less dead tissue behind (which reduces the risk of new adhesions forming). For ovarian cysts caused by endometriosis, excision also leads to better outcomes for both pain relief and future fertility.
Ablation can leave deeper tissue untouched and carries a higher risk of thermal damage to nearby structures like the ureters or bowel. Excision is considered the safer choice in those sensitive locations, though it requires more surgical skill. For deep infiltrating endometriosis, excision is the standard approach because the depth of the disease only becomes apparent once the surgeon begins cutting.
Growth Beyond the Pelvis
In rare cases, endometriosis tissue appears outside the pelvic area entirely. The most well-documented location is the chest cavity, where it tends to affect the diaphragm, chest wall, and lung tissue. Thoracic endometriosis most commonly causes recurrent collapsed lung (pneumothorax) that happens in sync with menstruation, typically within 72 hours of a period starting. About 90% of these cases occur on the right side. Symptoms include chest or shoulder pain, coughing, and shortness of breath.
Less frequently, thoracic endometriosis causes coughing up blood, also timed to the menstrual cycle. Interestingly, this particular symptom affects people about 8 to 10 years younger and occurs equally on both sides, suggesting it spreads through the bloodstream or lymphatic system rather than through the diaphragm. Hormonal therapy is often effective for these cases, with patients experiencing symptom improvement and no recurrence while on treatment.