controlled experiments are difficult to perform because of
the need for repeated surgical procedures to assess endometriotic
lesions over time. Animal models provide an invaluable
tool to study risk factors, prevalence and the natural
history of endometriosis, especially in those menstruating
non-human primates that develop the disease spontaneously.
The placebo group of women in placebo-controlled
trials provides some evidence of the natural history of the
disease. Endometriosis can both deteriorate and improve
spontaneously