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One in 10 women live with endometriosis, but we still don鈥檛聽know its exact cause. New聽evidence suggests that some聽cases may be present from birth, and that different subtypes of the condition may benefit from聽different treatments.

Endometriosis is diagnosed when endometrial cells from the聽lining of the uterus are found聽elsewhere in the body. When people with the condition menstruate, these patches of cells,聽wherever they are, bleed.

This is thought to be at least partly responsible for the debilitating pain people with endometriosis often experience, although we still聽don鈥檛 really know. Those affected often find it harder to聽conceive, too.

One of the more established ideas to explain what causes endometriosis suggests that these rogue cells are the product of 鈥渞etrograde menstruation鈥�. The idea is that some menstrual blood ends up being returned into the body, taking these uterine cells with it and enabling them to implant in the wrong place.

But this idea doesn鈥檛 explain all聽cases of endometriosis, says Katie Leap at the University of California, Los Angeles. 鈥淵ou can find endometriosis in fetuses, in聽men and in girls who haven鈥檛 menstruated,鈥� she says.

To learn more about the problematic patches of cells, known as lesions, Leap and her colleagues compared them with typical cells found in the uterus lining. Specifically, they looked at聽the cells鈥� epigenetic age: their聽biological age based on the presence or absence of chemicals that switch genes on or off.

The researchers analysed tissue聽samples collected from 60聽women, 35 of whom had endometriosis. They found that the biological ages of the tissues were different. While uterus cells had an age similar to the women鈥檚 chronological age, the cells from endometriosis lesions were around 16 years younger on average (bioRxiv, ).

The biological ages of these lesions were actually more like those of teratomas: tumours formed of multiple cell types, sometimes including hair and teeth, which can form during embryo development.

The researchers say this suggests some endometriosis lesions may also appear this early. As an embryo develops, cells that eventually form the uterus must move into the correct position. But 鈥渋t鈥檚 possible that some tissue gets left behind鈥�, says Leap. 鈥淭hat鈥檚 what happens with teratomas.鈥�

This doesn鈥檛 mean that retrograde menstruation doesn鈥檛聽happen, but Leap says it聽is聽unlikely to be the only cause of聽the different subtypes of endometriosis. The finding may improve diagnosis and treatment.

This evidence is suggestive, but聽not聽yet strong enough to confirm聽the idea, says Caroline Gargett at聽Monash University in Melbourne, Australia. The idea that endometriosis lesions are present at birth is 鈥渟till a theory waiting to be proven with more detailed studies鈥�, she says.