Endometriosis tracked more blood clots, from a low base

Endometriosis is usually discussed as a problem of pain and fertility. A Danish study in the European Heart Journal puts it somewhere less expected: on the list of things that raise the chance of a blood clot.

The comparison is about as complete as this kind of research gets. Every woman diagnosed in Denmark across 47 years, set against four times as many women who were not.

The gap is consistent, and it is narrow in the way that matters to any individual woman.

What endometriosis is

Endometriosis is a disease in which tissue like the lining of the uterus grows in other places in your body. These patches of tissue are called implants, nodules, or lesions, and they are most often found on the ovaries, the fallopian tubes and the outer surface of the uterus.

It is common and poorly understood. Researchers do not know what causes endometriosis, and it can affect anyone who menstruates. The researchers put the scale at around 10% of women of reproductive age.

The symptoms are the reason most people have heard of it. The main symptoms are pelvic pain, which often happens during your period, and infertility, with painful menstrual cramps, pain during or after sex, and heavy periods among the rest.

And there is no cure for endometriosis, but there are treatments for the symptoms: pain relief, hormone therapy that stops the ovaries from making hormones, and surgery.

Why a heart journal is interested

The link being tested runs through inflammation.

The researchers set it out plainly: emerging evidence suggests a link between endometriosis and systemic inflammation, potentially increasing the risk of venous thromboembolism, yet data on the association between endometriosis and VTE are lacking. Venous thromboembolism is the umbrella term for the two clot problems that concern them.

The first is a clot in a deep vein. Deep vein thrombosis, or DVT, is a blood clot that forms in a vein deep in the body, and most deep vein clots occur in the lower leg or thigh.

The second is what happens when that clot moves. A deep vein thrombosis can break loose and cause a serious problem in the lung, called a pulmonary embolism.

Long-running inflammation is a recognized route to both. Whether a specific inflammatory condition of the pelvis translates into clots elsewhere in the body, over a lifetime, is what nobody had measured.

How the Danish study was done

Denmark keeps national health registries that cover the whole population, which allows a study most countries cannot run.

It was a nationwide, registry-based cohort study including all women with endometriosis from 1977 to 2024, each matched 1:4 on birth and index year with women from the background population without endometriosis. That produced 63 999 women with endometriosis and 255 996 matched female controls.

Matching on year of birth and year of entry means the comparison groups are the same age at the same moment in history, which matters over a period this long: the hormonal treatments available in 1980 are not the ones available now.

The outcome was defined before the analysis. The primary outcome was clots as a composite of deep venous thrombosis and pulmonary embolism, with each component examined separately as well.

The two groups were not otherwise alike, and the paper says so. At baseline, women with endometriosis had a higher burden of comorbidities, had greater use of medications, and more often had a history of abdominal/gynecological surgeries.

What the clot rates were

The rate in women with endometriosis was 2.18 per 1000 person-years. In the matched women without it, 1.54 per 1000 person-years.

Both of those are small numbers, and the difference between them is smaller still: about six extra events for every ten thousand women each year. For one woman deciding how worried to be, that is the figure that counts.

Across a population it reads differently. Endometriosis affects roughly one woman in ten of reproductive age, the excess persists for decades, and the events in question are the serious kind: a deep vein thrombosis can break loose and cause a serious problem in the lung, called a pulmonary embolism.

The estimate held even after adjustment for relevant comorbidities and use of hormonal contraception, and it did something slightly odd in the process: the adjusted figure came out larger than the unadjusted one. Adjustment usually shrinks an association. Here it widened it, which points at measured differences that were pulling the estimate down.

Similar results were observed for both clot types, deep vein thrombosis alone and pulmonary embolism alone, at almost identical size. When two related outcomes move together by the same amount, it is a sign the analysis is picking up something coherent rather than a stray result.

The problem with how endometriosis gets diagnosed

There is one feature of this condition that complicates every study of it, and it is worth understanding.

Surgery is the only way to know for sure that you have endometriosis. The most common surgery to diagnose endometriosis is a laparoscopy, performed through a small cut near the belly button.

So the 63,999 women in this cohort are not women who have endometriosis. They are women who had an operation that found it. Everyone in the exposed group has had abdominal surgery, by definition of how they got there. And abdominal surgery is one of the classic triggers for a clot in a deep vein.

The abstract records that these women more often had a history of abdominal or gynecological surgeries, and describes the adjustment as covering relevant comorbidities and use of hormonal contraception. It does not say that surgical history was among the things adjusted for.

That may be answered in the full paper. From what is public, it is the single largest question hanging over the result, and it points the same way as another one: women under active gynecological care get scanned more, and a clot that is looked for is a clot that gets found.

What this cannot tell you

It is observational, so it establishes that these things travel together and not that one produces the other.

Registry diagnoses are as good as the codes entered, across a period in which both endometriosis and clot diagnosis changed substantially. A woman diagnosed in 1979 was found by different means than one diagnosed in 2023.

Denmark is one country with universal health coverage and a largely white population, and access to gynecological surgery there is not access elsewhere.

The paper is paywalled, so the authors’ own limitations section was not available for this piece.

What follows for women with endometriosis

The authors’ recommendation is modest and it is the right size for the finding: they highlight the importance of clinical awareness and consideration of thrombotic risk in women with a history of endometriosis.

That is a message aimed at doctors more than patients. It matters most at the moments when clot risk is already elevated and a decision is being made anyway: before surgery, during a long stretch of immobility, when a hormonal treatment is being chosen.

For a woman living with the condition, the practical version is short. The background chance of a clot remains low. Knowing the symptoms is free, and they are specific: pain or swelling in the part of the body affected, warmth and tenderness over the vein, and skin redness.

And there is a fair reading of this study that has nothing to do with clots. A condition that is routinely dismissed as period pain keeps turning up in registries attached to outcomes elsewhere in the body, which is an argument for taking it seriously as a systemic disease rather than a gynecological inconvenience.