Consequently, the figures obtained are not representative of the frequency of the illness in the general population. Any study on the frequency of endometriosis is potentially biased, as surgery is necessary to diagnose the pathology and selective factors can determine which patients are operated on. In spite of these problems, most researchers do not find any correlation between the occurence of endometriosis, age and parity. Some of them have found a relation with social class, race, tabagism or the use of an intra-uterine device. As concerns the literature, it can be concluded that all of the studies are influencd by the social-economic status of the patients, the age distribution, the availability of care, the accessibility of contraception, the cultural differences as concerns pregnancy, the behaviour towards menstruation and pain and the request for care.
Endometriosis is extremely rare prior to puberty and after the menopause.
The age distribution of women between puberty and the menopause is more of a reflection of the age when there is a higher demand for care, than the age at which the illness occurs, and the age at which the doctors are prepared to perform laparoscopic surgery on patients with pelvic pain. In countries where oral contraception is predominant, this can hide the symptoms of the illness. These symptoms could be revealed when the oral contraception is stopped, and the prevalence of the illness in the majority of western countries would become apparent. Animals and endometriosis Peritoneal endometriosis is discovered in patients with unexplained infertility. A significant number of hypotheses have been put forward to explain the relationship between endometriosis and infertility.