Association of endometriosis and spontaneous abortion: effect of control group selection.

Endometriosis has been associated with an increased incidence of spontaneous abortion, compared with the abortion rate of the general population. To assess whether a separate control group would affect these conclusions, we studied 139 consecutive infertility patients with laparoscopically proven endometriosis to determine the incidence of spontaneous abortion. Ninety-five of these patients underwent conservative surgical resection of endometriosis, and 44 patients opted for expectant management. There was no significant difference between these two groups in average age, duration of infertility, or proportion of patients with primary infertility. The average spontaneous abortion rate before diagnosis for all patients was 63.1%. After surgical treatment, the abortion rate dropped to 0% (P less than 0.000001) for all stages of disease. However, even in those patients who received expectant management only, the abortion rate fell to 16.7% and 21.4% for mild and moderate endometriosis, respectively (P less than 0.001). These results suggest that the spontaneous abortion rate in untreated endometriosis may not be as high as previously reported and may not be significantly different from the rate in the general population. The data also emphasize the need for well-defined control groups when assessing the effects of a treatment regimen.