New nerve growth may explain why the condition endometriosis is painful, US scientists believe.
Two million women in the UK have endometriosis
Endometriosis occurs when the tissue which normally lines the uterus is found elsewhere in the body, causing pain and sometimes infertility.
Professor Karen Berkley and colleagues at Universities in Florida and Ohio say this 'trespassing' lining grows its own nerve supply which transmits pain.
The study appears in Proceedings of the National Academy of Sciences.
Doctors have known that endometriosis is a painful condition, which usually involves the pelvis.
The pain occurs during a woman's period, when the endometrial lining in her womb breaks down and sheds off.
Each month the tissue outside of the uterus, under normal hormonal control, also breaks down and bleeds in the same way as the womb lining but has no way of leaving the body.
This leads to inflammation and pain.
But the exact mechanism behind the pain is poorly understood.
To study this, the US team operated on rats to give them a condition mimicking endometriosis by transplanting endometrial tissue from the womb into the abdomen.
They later removed the transplanted tissue to see what had happened to it.
The tissue had grown its own nerve supply that could transmit information about injury and pain to the brain.
The researchers believe this nerve supply might also help the abnormally-located tissue survive and grow by setting up its own blood supply.
They cautioned that the findings only apply to rats, but said the same might be true in women.
If it is, this might lead to new treatments, they said.
Professor Berkley said: "It opens up a whole new way of thinking about how the disease comes about and is maintained and how its symptoms come about.
"It's bringing about the possibility of treating with something other than hormonal treatment or surgical treatment," she said.
The researchers are now doing studies looking at human tissue.
Sanjay Vyas, consultant obstetrician and gynaecologist at Southmead Hospital in Bristol, said: "It's an interesting lead and it's good news.
"If you can understand how the disease causes symptoms it can help target treatment.
"The downside is it is in rats not humans and within the rats this is not spontaneous endometriosis so there might be subtle differences," he said.
Jeremy Wright, president of the British Society for Gynaecological Endoscopy, said it was an "elegant" study but he doubted the nerves could regulate the growth of blood vessels in endometriosis.
He had similar concerns to Mr Vyas about whether the findings would apply to humans.
Chief executive of the National Endometriosis Society Robert Music said: "This sounds interesting, although obviously it is only on rats. There is a real need for research into endometriosis, which affects two million women in the UK."