Seventy-seven of 226 pregnancies (34%) conceived prior to conservative surgical treatment of endometriosis ended in first-trimester spontaneous abortion. Mild endometriosis was associated with a significantly greater proportion of abortions (49% in 87 pregnancies) than moderate (25% in 32 pregnancies) or severe disease (24% in 107 pregnancies). Following operation, only 7 of 76 pregnancies (9%) conceived by the same group of patients ended in spontaneous abortion. In primary infertility patients, 17 of 128 pregnancies (13%) ended in abortion following operation. The postoperative abortion rates were not significantly different between degree of severity of endometriosis. Speculation is presented on the mechanism of the increased preoperative abortion rate in patients with mild endometriosis.
Wheeler, J. M., Johnston, B. M., & Malinak, L. R. (1983). The relationship of endometriosis to spontaneous abortion. Fertility and Sterility, 39(5), 656–660. https://doi.org/10.1016/S0015-0282(16)47061-5