Endometriosis causes swelling and pain
Scientists have identified a possible cause of the painful womb condition endometriosis.
The team at the University of Liverpool believe over-production of an enzyme, telomerase, which plays a role in cell division may be responsible.
Endometriosis results in patches of the womb's inner lining growing in other parts of the body.
It is hoped the Human Reproduction study may lead to new ways to diagnose and treat the condition.
Endometriosis, which affects around two million women in the UK alone, can cause severe pain, heavy periods and infertility.
Uusally diagnosed between the ages of 25 and 40
Most commonly occurs in the fallopian tubes, ovaries, bladder, the bowel, the intestines, the vagina and the rectum
Endometriosis cells behave in the same way as those lining the womb, so every month they grow during the menstrual cycle and shed blood
Normally before a period, the womb lining thickens, then - if no pregnancy results - breaks down and bleeds
Endometrial tissue behaves the same way, but has no way of leaving the body
The trapped tissue leads to swelling, pain and bleeding
However, the cause of the condition remains unknown.
Telomerase maintains the length of key structures called telomeres, which help determine how long a cell can keep dividing.
Telomeres are essentially protective caps which stop the cell's chromosomes from fraying and coming apart when they divide.
With every division, the telomeres become shorter, and eventually the cell is unable to divide any more.
Telomerase is usually produced at the start of a woman's menstrual cycle when cell division is important.
But at the end of the cycle, when embryo implantation becomes the priority, telomerase production tends to fall off.
End of cycle
However, the Liverpool team found that in women with endometriosis telomerase continues to be produced by the cells of the inner lining of the womb even in the later stages of the menstrual cycle.
Consequently, the cells have unusually long telomeres, and the ability to divide uncontrollably, acquiring the ability to spread and survive outside the uterus.
Lead researcher Dr Dharani Hapangama said: "Women who have endometriosis express this enzyme in both the early and late stages of the menstrual cycle which means that the cells will continue to divide and lose their 'focus' in supporting the establishment of a pregnancy.
"As a result the lining of the womb may be more hostile to an early pregnancy, and the cells that are shed at this late stage in the menstrual cycle may be more 'aggressive' and more able to survive and implant outside the uterus, causing pain in the pelvic or abdomen area."
A spokesperson for the charity Endometriosis UK said: "This is an interesting piece of research which highlights differences between women with endometriosis and those without.
"However, whilst we welcome any research which increase our understanding of this condition we appreciate that this is still a long way from providing a complete understanding of the condition or indeed new therapies.
"We hope that these observations will be developed and improve our understanding and ultimately lead to new treatments."