LONDON (AP) - Women with endometriosis - a leading cause of infertility in which tissue from the womb lining grows elsewhere in the body - are much more likely to suffer from rheumatoid arthritis, lupus, chronic fatigue syndrome, fibromyalgia and allergies, new research has found. The study, published this week in the journal Human Reproduction, is the first to document something that has been noticed by many women with the painful disorder. The researchers urged doctors to look for the other diseases in women when diagnosing endometriosis, which afflicts between 8 percent and 10 percent of women of childbearing age. The cause of endometriosis, as well as of the other diseases, remains unknown. The new research suggests an immune system abnormality may underlie all these conditions, said Warren Nothnick, a University of Kansas professor of obstetrics and gynecology who was not connected with the study. ``What is the underlying factor, the commonality, between all of these diseases? If we can find out what the one factor is, or group of factors, we can target that and hopefully come up with a way to treat not only the endometriosis, but also some of the other diseases,'' Nothnick said. His own work, as well as that of other scientists, suggests that the malfunctioning of certain immune system chemicals called cytokines may be a common link. Assuming endometriosis occurs before the other diseases, it may also be possible one day to spare women with endometriosis from developing some of the additional diseases, Nothnick said. The study, conducted by scientists at the U.S. National Institute of Child Health and Human Development, George Washington University and the Endometriosis Association, involved 3,680 women with endometriosis. The scientists found that 20 percent of the women had more than one other disease. A third of the women who had other diseases had fibromyalgia or chronic fatigue syndrome, and some of those women also had other autoimmune or hormone diseases. Chronic fatigue syndrome was more than 100 times more common than among the general U.S. female population. Hypothyroidism, which involves an underactive thyroid gland and causes mental and physical slowing, was seven times more common. Fibromyalgia, which is characterized by widespread body pain and tiredness, was twice as common among the women with endometriosis. Autoimmune inflammatory diseases - systemic lupus, rheumatoid arthritis and multiple sclerosis - also occurred more frequently than normal. Rates of allergic conditions were higher, too. While allergies occur in 18 percent of U.S. women, they occurred in 61 percent of the women with endometriosis. The rate climbed even higher if the women had additional diseases. The study also confirmed previous findings that there is typically a 10-year gap between the time women first get the pelvic pain and the diagnosis of endometriosis. The most common symptom of endometriosis is pain, especially excessive cramps during menstrual periods or pain during or after sex. Infertility occurs in about 30 to 40 percent of women with endometriosis. Endometrial tissue outside the uterus responds to the menstrual cycle similarly to the way it responds in the uterus. At the end of every cycle, when hormones cause the uterus to shed its endometrial lining, endometrial tissue growing outside the uterus also breaks apart and bleeds. However, unlike menstrual fluid from the womb, which is flushed from the body during menstrual periods, blood from the misplaced tissue has no place to go. Tissues surrounding the area of endometriosis may become inflamed or swollen. It is one of the most complex and least understood gynecological diseases and, despite many theories, the cause remains unknown. One theory is that during menstruation some of the menstrual tissue backs up through the fallopian tubes into the abdomen, where it implants and grows.