What causes endometriosis

Knowing what endometriosis is leaves a harder question open: why does it happen, why does it look so different from one woman to the next, and why is it so often found late? These are the questions where medicine is still working, and where honesty matters more than certainty.

This article gathers what is known, and what is not. It is written to inform, and it does not replace a consultation with a physician.

There is not one endometriosis, but several

The first thing to understand is that endometriosis is not a single, uniform disease. It settles in different places, in different proportions, and follows a different course in each woman. Two people with the same diagnosis can have almost nothing in common in how the disease behaves.

This variability is not a detail. It shapes everything that follows: why a single explanation has never been enough, why symptoms are so hard to map onto lesions, and why treatment has to be tailored rather than standardised.

Where does it come from?

The origin of endometriosis is still debated. Several theories exist, each accounting for part of the picture, and none for all of it.

The oldest and most widely cited is retrograde menstruation, described by the gynaecologist John Sampson in 1927. During menstruation, some blood flows backward through the fallopian tubes into the pelvis, carrying fragments of endometrium that settle and grow. The difficulty is that this backward flow happens in almost every woman, while only about one in ten develops endometriosis. So the reflux alone cannot be the whole story. Something decides who is affected, and the immune system is a leading suspect: in most women it clears these stray cells, and in some it appears not to.

A second theory, metaplasia, proposes that certain cells already present in the abdomen transform in place into endometrial tissue, without any need for them to travel there.

A third, sometimes called embryonic rest, suggests that remnants of tissue laid down before birth, scattered through the body during development, can later give rise to lesions. This would help explain the rare cases found far from the pelvis, in the diaphragm or the chest. Lesions have even been described that travel through the lymphatic and blood vessels to reach distant sites such as the navel.

A disease with many causes

Most researchers now believe the variety of forms reflects a variety of causes working together, rather than one mechanism acting alone. Several threads are being followed at once.

There is a genetic and epigenetic predisposition: the disease runs in families, and the way genes are switched on and off appears to differ in affected women. There is the immune system, which may fail to clear misplaced cells and may sustain the inflammation that follows. There is hormonal sensitivity, since the disease is fuelled by oestrogen. And there are environmental exposures, particularly endocrine-disrupting chemicals, which resemble natural hormones closely enough to interfere with them.

More recent work has begun to examine the microbiome, the community of bacteria living in and around the reproductive tract, as a possible contributor. This is a young line of inquiry, drawn from small studies, and its conclusions are not yet settled.

Are there risk factors?

Some factors are associated with a higher likelihood of developing the disease: an early age at first periods, a family history of endometriosis, and certain environmental and lifestyle exposures. These raise the odds; they do not cause the disease on their own, and most women who have them never develop it. They are signals worth noting, not verdicts.

Why is it found so late?

A diagnosis of endometriosis is often reached only after several years. Studies place the average delay somewhere between seven and ten years, with a string of medical visits along the way. In the United States, the figure is commonly cited at five to eight years.

The reasons are layered, and none of them rests on a single person. Severe menstrual pain is still widely treated as ordinary, which makes it easy to set aside, by patients and clinicians alike. The disease imitates other conditions, from digestive disorders to bladder complaints. And the imaging that can reveal it, pelvic MRI, is one of the most demanding examinations to interpret in all of radiology. The specialisation it calls for is rare, and the healthcare system does not always create the conditions for it.

This is why the diagnostic question is, at heart, a question of reading. The images are increasingly available. What is scarce is the time, the training, and the attention that a precise reading requires.

What is still unknown

It is worth being clear about how much remains open. The origin of the disease is debated. So is the link between the lesions and the pain, which often do not match in the way one would expect. So is why the disease progresses in some women and stays quiet in others, and why it responds to treatment in some and resists it in others.

Several research efforts are underway, including work on genetics, the immune system, and non-invasive ways to detect the disease earlier. A clearer understanding of where endometriosis comes from would be the foundation for better treatment. That foundation is still being built.

Geneviève is a diagnosis program for endometriosis and complex gynaecological conditions. It combines advanced pelvic MRI with interpretation by physicians who specialise exclusively in gynaecological imaging, supported by AI trained on the world's largest dataset in this field.

This article is for information and does not replace a consultation with a physician.