It has never been shown by any biopsy-controlled study that pregnancy eradicates or cures endometriosis. In fact, there are several studies which have shown a predominance of previously fertile women among patients diagnosed with endometriosis.
Haydon found that 58% of his endometriosis patients had previously delivered babies. Counsellor found a 61.3% crude previous birthrate among 737 married patients with endometriosis. Bennet found a previous crude birthrate of 88% in his married patients with endometriosis as their only gynecologic pathology. Dougherty found that 87% of his endometriosis patients were parous, leading the author to wonder whether pregnancy caused endometriosis. Andrews and Larsen found a crude previous pregnancy rate of 72% among their married patients with endometriosis. Redwine found that 65% of his newly-diagnosed patients with endometriosis had been pregnant, and 51% had delivered babies. These rates would be even higher if corrected for patients not attempting pregnancy. It should also be remembered that so many women give birth by caesarion section these days, which causes scar tissue, which causes its own sets of problems, one of which may be the spread and cause of post-natal endometriosis.
Certainly if pregnancy were a cure for endometriosis, none of these fertile patients would have had the disease. Instead, it is obvious from the reported literature that there have been hundreds of patients with endometriosis who have previously been pregnant. The problem arose because the studies mentioned above were not the first studies commenting on previous fertility and endometriosis. The earlier studies by Sampson and Meigs had found a crude previous fertility rate of around 40% in married endometriosis patients. (No mention was made of other fertility factors, or whether the couples were even trying to conceive.) It has to be mentioned though that the data on the endometriosis staus of these women prior to their pregnacies are not clear and one doesn't know if they had had endometriosis before and other factors involved that could have had an influence.
The conclusion was made from these early studies that pregnancy seemed to be protective against endometriosis, and, the corollary followed that infertility was due to endometriosis. This all occurred as a result of Berkson's fallacy - the observation in hospitalized patients of a possibly spurious relationship between disease states or symptoms, followed by the conclusion that this relationship was cause and effect. The notion was proposed, seemed logical, and was accepted as an accomplished fact which has gone unquestioned to this day. This is now recognized as a classic case of the misdirection which can be introduced by selection bias.
Since so many studies have been published which show a predominance of fertility among endometriosis patients, one might reasonably ask, as did Dougherty, whether pregnancy causes endometriosis, or whether endometriosis causes fertility. While these types of proposals may seem outrageous, consider that their opposites (that infertility causes endometriosis or that endometriosis causes infertility) have not only been proposed, but have been accepted as scientific fact and offered as a viable treatment for generations of women with endometriosis. Ascribing causality solely on the basis of an observed relationship is a truly outrageous abuse of the scientific process which would not be allowed in an introductory course on epidemiology.
Electrocoagulation of endometriosis is performed by touching a metal electrode to the disease and destroying it where it lies. The drawback is that the surgeon has no way of knowing whether the disease is being completely destroyed. This approach may also risk injury to underlying vital structures. Electroexcision of endometriosis, on the other hand, separates endometriosis from vital structures using blunt dissection and short bursts of high density current which cut tissue quickly.
Birth control pills (BCPs) seemed a logical therapy which might duplicate the imagined protective effect of pregnancy. Pseudopregnancy therapy of endometriosis with BCPs had its impetus from a patient without proven endometriosis who was thought to have been cured of her "disease" by pregnancy. Ironically, subsequent studies which found surgical treatment slightly more effective than BCPs or which studied response of the uterine lining, not endometriosis, were used to support the use of BCPs for endometriosis, just another abuse of science.
The notion of an "assumed fact" used in the scientific process was first mentioned in the early development of BCP therapy. It is now recognized that in order to study a specific therapy, it is incorrect to combine it with another therapy and ignore the added therapy. Also, the inaccuracy invited by studying any disease process by studying something else other than the disease should be obvious.
This may seem confusing, which it is, but the bottom line is that research has and always will be manipulated to influence the outcome in favour of the treatment option being studied.