Causes of endometriosis: what the largest study to date shows

If you are wondering where your endometriosis comes from, you are not alone. And for a long time the honest answer was: no one knows exactly. That can be wearing. You are in pain, maybe for years, and no one can really explain why. But something big is moving in our understanding of the causes of endometriosis. The largest study on this condition to date has analysed the genetic data of 1.4 million women. Its most important message to you: your symptoms are real, biologically grounded and certainly not imagined.

What is endometriosis?

Endometriosis is a chronic condition in which tissue similar to the lining of the uterus grows outside the uterus, for example on the ovaries, the peritoneum or other organs. This tissue responds to your cycle, it bleeds and becomes inflamed, and that can cause severe pain.

It is not rare. About one in ten women of reproductive age is affected. Even so, it takes seven to ten years on average to reach a diagnosis. So if you have felt that no one listened to you for a long time, that is not your fault. It is a well known problem.

One sentence matters to us here: severe period or pelvic pain that regularly holds you back is not normal. It is not simply part of the deal, and you do not have to put up with it.

Causes of endometriosis: what lies behind it?

The short answer: the causes of endometriosis cannot be reduced to a single trigger. For a long time the so called implantation or retrograde menstruation theory was the main explanation. During your period, some blood flows back through the fallopian tubes into the abdomen and carries lining cells with it. But that does not explain everything, because this backflow happens in almost everyone who menstruates, and still not everyone develops endometriosis.

Today the view is that several factors work together: your genetic predisposition, hormones, the immune system and environmental influences. It is exactly this interplay that makes a simple answer so hard. And this is where the new research comes in.

What role do hormones, the immune system and the environment play?

Alongside the genes, hormones play a major part in the causes of endometriosis. Endometriosis depends heavily on the hormone estrogen. The tissue grows under its influence and responds to the ups and downs of your cycle. That explains why symptoms often get worse around your period and why some hormonal treatments can help at all.

The immune system is involved too. Normally the body clears away stray lining cells in the abdomen. In endometriosis this clean up seems to work less well, so tissue can settle and become inflamed. On top of that come environmental influences that are still being studied. None of these factors explains the condition on its own. Only their interplay creates the full picture. That is exactly why the search for a single cause is so hard, and why it helps that the new research looks at many pieces of the puzzle at once.

What does the largest study to date show?

In April 2026 the journal Nature Genetics published the largest genetic study of endometriosis so far. Researchers from Yale, the University of Barcelona and other institutes analysed the genetic data of around 1.4 million women, including more than 100,000 with endometriosis. For understanding the causes of endometriosis, this is a big step.

They found 80 regions in the genome that influence risk, 37 of them previously unknown. Five of these new regions are also linked to adenomyosis, a related condition in which similar tissue grows into the muscle layer of the uterus.

The most important finding, though, is not a single number. The data show that several biological processes play a role in endometriosis at the same time: inflammation and an altered immune response, the remodelling and growth of tissue, and the formation of new blood vessels. So endometriosis is probably not one single disease with one cause, but rather a bundle of pathways that lead to the same diagnosis.

Is endometriosis hereditary?

In part, yes, and this is a question that worries many, especially with a daughter in mind. Twin studies estimate that about half of the risk lies in the genes. If your mother, sister or daughter is affected, your own risk is about five to ten times higher than average.

But this matters: what is passed on is a predisposition, not a certain illness. There is no single endometriosis gene that you inherit. Many small genetic influences add up, and whether they actually turn into symptoms also depends on hormones, the immune system and the environment. The genes are one part of the causes of endometriosis, but not the whole story. A family history is therefore a clue you can raise with your doctor, not a fate.

And one more thing: you did not cause this condition through anything you did or did not do. Neither your diet nor your lifestyle is to blame.

Why the finding that it is not one disease matters so much

You may know this feeling: a treatment works well for a friend but barely helps you, even though you share the same diagnosis. The new research offers an explanation. If different biological processes lie behind endometriosis, then it makes sense that not everyone responds to the same therapy in the same way.

For the future this is good news. The better the different forms can be told apart, the more precisely diagnosis and treatment can be tailored to you, instead of treating everyone the same.

What does this mean for new treatments?

The more precisely the causes of endometriosis are known, the more targeted the search for medicines can be. When you know which biological processes are involved, new paths open up. Researchers see the possibility, for example, of testing already approved drugs from other fields for their effect on endometriosis, such as from cancer medicine. A drug that could ease both pain and an accompanying low mood is also being discussed.

As hopeful as that sounds, the honest part remains: these are prospects, not finished therapies that will be on the pharmacy shelf tomorrow. It usually takes years for such leads to become an approved treatment. False promises do not help you, but a realistic view does.

What can you do now?

Even if the big answers still have to come from research, there are things that matter for you today:

• Take your pain seriously. If it regularly limits you, it is a reason for an appointment, not a sign of being oversensitive.
• Mention a family history. If close relatives have endometriosis, that is important information for your doctor.
• Keep a short pain and cycle diary. It makes it easier to show the pattern of your symptoms.
• Get a second opinion if you do not feel taken seriously. You have a right to answers.

Even if the causes of endometriosis are not yet fully understood, the new study shifts something important. It will not change your treatment overnight. But it shows that endometriosis is not a vague women's issue but a biologically tangible condition that is being taken seriously and researched. And you are not alone with your questions.

This article is for general information only and does not replace medical advice. If you have severe, unusual or persistent symptoms, please see your gynaecologist.