We have for the first time demonstrated the existence of a hereditary component to endometriosis that can be traced beyond first-generation relatives

Dr Kari Stefansson, deCODE

Keyhole surgery is currently the only way to confirm or rule-out the condition, and the average time between the onset of symptoms and diagnosis is about 10 years.

The scientists said women who have endometriosis in their families may want to consider pregnancy earlier rather than later in their reproductive life, because the condition tends to get worse with time.

Endometriosis occurs when the cells normally found lining the uterus are also found in other areas of the body, usually within the pelvis.

The tissue reacts as it would if it was still in the uterus, and every month, under normal hormonal control, it is built before breaking down and bleeding in the same way as the lining of the uterus.

Because, unlike a period, this bleeding has no way of leaving the body, inflammation, pain and the formation of scar tissue occur.

Up to 5% of women are thought to suffer from endometriosis in their reproductive years.

Genetic database

The Icelandic study is the first to analyse how often endometriosis occurs across a population.

Scientists from deCODE genetics and Iceland's National University Hospital, in Reykjavik, used the company's anonymised genetic computer database of hundreds of thousands of present and past generations of Icelanders.

They compiled a list of all 750 women in Iceland who had been diagnosed with endometriosis between 1981 and 1993.

The list was then compared with the database to show family connections.

Other research has reported an increased risk of endometriosis between first-degree relatives with the disease, but the Icelandic research is believed to be the first to demonstrate the link between cousins.

The researchers said the study also showed the genetic factors involved in endometriosis can be inherited through paternal as well as maternal lines.

DNA test

Professor Reynir Geirsson, chairman of the Department of Obstetrics and Gynaecology at the National University Hospital, said: "Sisters share half of their genomes, but cousins share 12.5% and the difference seen for the latter group may therefore more accurately reflect the genetic liability."

Dr Kari Stefansson, Chief Executive Officer of deCODE and co-author of the article, said: "We have for the first time demonstrated the existence of a hereditary component to endometriosis that can be traced beyond first-generation relatives.

"The study is also important as the basis for a genome-wide scan to identify key genes that contribute to the disease.

"We are advancing in this effort, which we hope will contribute to the development of a DNA-based diagnostic test.

"Such a test would assist in diagnosing the disease and in identifying women at particular risk of endometriosis, without the need for invasive procedures."

Professor David Taylor, chairman of the research advisory committee of WellBeing, a health research charity , said: "Endometriosis has been recognised to have an inherited component, and this excellent study from Iceland has demonstrated that it not only occurs in sisters of women with the disease, but also in their cousins.

"The challenge now is to find out which specific genes cause the disease. WellBeing is already funding a research group in Oxford to do this."

The research is published in the journal Human Reproduction.