Endometriosis develops when tissue similar to the uterine lining grows outside the uterus, but how it gets there involves several overlapping biological processes rather than a single cause. The condition affects roughly 10% of women of reproductive age worldwide, about 190 million people, yet it takes an average of 7.5 to 10 years from symptom onset to diagnosis. Understanding the mechanisms behind it helps explain why it’s so persistent and so difficult to catch early.
Menstrual Blood Flowing the Wrong Direction
The oldest and most widely cited explanation is retrograde menstruation. During a normal period, the uterus sheds its lining and pushes it out through the cervix. But in many people, some of that menstrual blood flows backward through the fallopian tubes and spills into the pelvic cavity. The endometrial cells in that blood can then land on the surfaces of nearby organs, including the ovaries, bowel, and the tissue lining the pelvis.
Here’s the problem with this theory on its own: retrograde menstruation happens in up to 90% of menstruating people, yet only about 10% develop endometriosis. So backward flow is likely a necessary first step for many cases, but something else has to go wrong for those cells to actually take root and grow. That “something else” involves the immune system, hormones, and genetics.
Cell Transformation Outside the Uterus
A second theory proposes that cells already present in the pelvic cavity can transform into endometrial-like tissue on their own. This idea, called coelomic metaplasia, is based on the fact that both the uterine lining and the membrane covering pelvic organs originate from the same type of embryonic cells. Under certain conditions, those pelvic surface cells may essentially “switch” into endometrial cells, forming glands and supportive tissue identical to what lines the uterus.
This explanation is especially useful for understanding ovarian endometriosis (endometriomas), because the surface layer of the ovary has a particularly high potential for this kind of cellular transformation. It also helps explain the rare cases of endometriosis found in people who have never menstruated or, in extremely rare instances, in men receiving estrogen therapy.
Spread Through Blood and Lymph Vessels
Endometriosis occasionally shows up in places far from the pelvis, including the lungs, diaphragm, and brain. Retrograde menstruation can’t account for that. Research supports a third pathway: endometrial cells entering the bloodstream or the lymphatic system and traveling to distant sites, much the way cancer cells metastasize.
Studies have found endometrial-like cells inside uterine-draining lymph nodes and various other pelvic lymph nodes. Women with endometriosis also show increased lymphatic vessel density in their uterine lining, which likely makes it easier for endometrial tissue to enter lymphatic circulation in the first place. Once in the lymph or blood, these cells can implant and grow in remote locations, contributing to high recurrence rates even after surgical treatment.
Why the Immune System Fails to Stop It
Normally, when foreign tissue lands somewhere it doesn’t belong, the immune system destroys it. In endometriosis, that cleanup process breaks down at multiple levels.
Macrophages, the immune cells responsible for engulfing and digesting cellular debris, lose much of their ability to do so. High levels of iron from menstrual blood overwhelm them. Chemical signals in the pelvic fluid suppress their activity further. And the endometrial tissue itself produces a surface molecule that essentially tells macrophages “don’t eat me,” blocking the immune response directly.
Natural killer cells, another frontline defense, are also suppressed. In people with endometriosis, these cells have fewer activating receptors and more inhibitory ones. Platelets and inflammatory chemicals in the pelvic fluid further dampen their ability to attack misplaced tissue. The result is an immune environment that tolerates the implants instead of eliminating them.
Making things worse, macrophages that do accumulate around lesions often shift into a repair-oriented mode that actually helps the endometrial implants grow. They release factors that stimulate new blood vessel formation, feeding the lesions with their own blood supply and helping them become self-sustaining.
How Estrogen Fuels Lesion Growth
Estrogen is the primary driver that keeps endometriosis lesions alive and growing. The uterine lining thickens in response to estrogen during every menstrual cycle, and endometrial implants outside the uterus respond to the same hormone in the same way. They swell, break down, and bleed with each cycle, but unlike the uterine lining, they have no way to exit the body.
Endometriotic lesions go a step further by producing their own estrogen. The cells in these implants contain enzymes that convert other hormones into estrogen locally, creating a self-reinforcing loop. Platelets that accumulate around the lesions amplify this process even more, boosting estrogen production while simultaneously promoting new blood vessel growth and protecting the implants from cell death. This local estrogen production is one reason endometriosis can persist even when systemic hormone levels are suppressed with medication.
The Inflammatory Cycle That Causes Pain
Endometriosis lesions trigger a chronic inflammatory response in the surrounding tissue. The implants and the immune cells they attract release a cascade of inflammatory chemicals, including prostaglandins, TNF-alpha, and several interleukins. These molecules serve multiple destructive roles: they recruit still more immune cells to the area, stimulate the growth of the lesions, and directly irritate nearby nerves.
Prostaglandin E2 plays an outsized role. Produced by the endometriotic glands themselves, it promotes inflammation, amplifies pain signaling, and stimulates cell growth all at once. Over time, nerve fibers actually infiltrate the lesions, creating direct wiring between the implants and the pain-sensing nervous system. This nerve infiltration is a major reason endometriosis pain can be so severe and so difficult to manage with standard painkillers.
The inflammation also generates reactive oxygen species, which damage surrounding tissue and amplify the inflammatory signals further. This creates a feedback loop: inflammation promotes lesion survival, lesion survival produces more inflammation, and the cycle deepens with each menstrual period.
Genetic Risk Factors
Endometriosis runs in families, and genome-wide studies have identified specific genetic variations that increase susceptibility. One of the most studied is a gene called WNT4, which plays a central role in the development of the female reproductive system. Several variations in this gene are significantly associated with endometriosis risk, with some increasing the odds by roughly 30 to 44%.
People with endometriosis also show reduced expression of WNT4 in their uterine lining compared to those without the condition, suggesting the gene’s activity level, not just its structure, matters. WNT4 is far from the only gene involved. Multiple genetic regions have been linked to the condition, each contributing a small amount of risk. No single gene causes endometriosis, but inheriting several of these variations together can make it substantially more likely.
Staging: From Minimal to Severe
Once identified, endometriosis is classified into four stages: minimal, mild, moderate, and severe. The staging system assigns points based on how deep the lesions penetrate tissue, how large they are, and whether dense scar-like adhesions have formed around the fallopian tubes, ovaries, or the space behind the uterus. The points are totaled to determine the stage.
One important thing to know: stage does not reliably predict pain. Someone with stage I (minimal) disease can experience debilitating symptoms, while someone with stage IV (severe) may have relatively mild discomfort. The location of the implants, their proximity to nerves, and the degree of nerve infiltration often matter more than the sheer volume of disease. This disconnect between what doctors see on imaging or during surgery and what patients actually feel is one of the reasons diagnosis takes so long.