What is endometriosis?

Endometriosis is one of the most common conditions in women's health, and one of the least understood. It affects close to one in ten women and girls of reproductive age, around 190 million people worldwide. Despite how widespread it is, it remains under-recognised, and a clear diagnosis often takes years to reach.

This article explains what endometriosis is, where it sits, and the forms it takes. It is written to inform, and it does not replace a consultation with a physician.

A definition

The inside of the uterus is lined with a tissue called the endometrium, which thickens and sheds with each menstrual cycle. Endometriosis is the presence of tissue resembling this lining in places it does not belong, outside the uterine cavity.

Wherever it settles, this tissue keeps the behaviour of its origin. It responds to the hormones of the menstrual cycle, thickening and bleeding in place. With no way to leave the body, the blood and inflammation it produces stay trapped, irritating the surrounding tissue and, over time, forming adhesions and scarring.

The tissue is endometrial in nature, glands and supporting stroma together. As it adapts to its new surroundings it changes slightly, so it is never quite identical to the lining inside the uterus. This is why it is often described as endometrial-like.

There is not one endometriosis, but several

The disease does not develop the same way from one woman to the next. Physicians today describe three main forms, which can appear on their own or together.

Superficial endometriosis sits on the surface of the peritoneum, the membrane that lines the abdominal cavity.

Ovarian endometriosis takes the form of a cyst on the ovary, filled with thick, dark fluid. It is sometimes called a chocolate cyst, or an endometrioma.

Deep endometriosis infiltrates more than five millimetres beneath the surface, often reaching the uterosacral ligaments, the bowel, the bladder, or the ureters.

What endometriosis feels like

Endometriosis is most often a disease of pain, though its symptoms reach well beyond the pelvis and well beyond the days of menstruation.

The pain has recognisable forms. Periods can be severe enough to interrupt ordinary life, beyond what painkillers settle. Many women feel pain during or after sex, felt deep rather than at the surface. Some have pain when passing stool or when urinating, often worse around their period. And for many, the pain becomes chronic, present on ordinary days with no relation to the cycle at all.

Around these runs a wider set of signs: heavy or irregular bleeding, bloating and digestive trouble, a fatigue that rest does not lift, and, in some women, difficulty conceiving. The disease is also closely associated with anxiety and low mood, which years of unexplained pain can deepen.

What ties these together is timing. Symptoms that follow the rhythm of the menstrual cycle, returning and easing month after month, are among the clearest signs that something more than ordinary period pain may be at work. 

Fertility is the exception to that rhythm. For some women, difficulty conceiving is the first sign of endometriosis, and occasionally the only one, arriving with no pain at all. The disease is found in a large share of women investigated for infertility, which is why it is often discovered during a fertility workup rather than because of symptoms. This does not put motherhood out of reach: most women with endometriosis who want to conceive do, with or without help.

We look at each of these, fertility included, in dedicated articles.

Severity is not correlated with pain

The form the disease takes does not predict how much it will hurt. Superficial lesions can be intensely painful, partly because of the density of nerves where they settle, while more extensive disease is sometimes nearly silent. This is one of the reasons endometriosis is so difficult to read from symptoms alone, and one of the reasons it is so often underestimated.

Endometriosis is benign in the strict medical sense, meaning it does not threaten life. It can also be profoundly disabling, severe enough to disrupt work, intimacy, and daily life. Both things are true at once.

How common it is, and who it affects

Endometriosis affects around one in ten women and girls of reproductive age. It can begin with the very first periods. Many women who are severely affected describe years of violent menstrual pain dating back to adolescence. It is not a disease of adulthood alone.

And yet the diagnosis is often reached only after several years, a delay we look at closely in a companion article. The earlier the disease is recognised, the more can be done to limit its progression and protect quality of life.

A systemic disease

For most of its history, endometriosis was defined as a gynaecological disease, a problem of the pelvis and the reproductive organs. That definition is now considered too narrow. In an influential 2021 review in The Lancet, Hugh Taylor and colleagues argued that this older description is outdated, and that endometriosis is better understood as a systemic disease rather than one confined to the pelvis.

Their case is that the disease reaches well beyond where its lesions sit. It alters metabolism in the liver and fat tissue, drives inflammation throughout the body, and changes activity in the brain in ways that can heighten pain and affect mood. The lesions themselves are found most often in the pelvis, on the peritoneum, the ovaries, the ligaments that support the uterus, the bowel and the bladder, and in rarer cases they reach as far as the diaphragm or the chest.

The same review is careful to add that these wider effects are still poorly understood, and that the full reach of the disease is not yet fully recognised. This is an understanding still taking shape. But it matters, because it recasts endometriosis as a condition of the whole body, and many of the symptoms that come with it, such as fatigue and trouble concentrating, as connected to the disease rather than separate from it.

Geneviève is a diagnosis program for endometriosis and complex gynaecological conditions, built around a single belief: that every woman deserves a precise, thorough answer, delivered with care.

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