Effect of pregnancy on surgically induced endometriosis in cynomolgus monkeys

There are few objective data to support the clinical impression that pregnancy exerts a beneficial effect on endometriosis. In this study, we examined the progression-regression of endometriosis in pregnant and nonpregnant monkeys. Endometriosis was surgically induced in 16 monkeys and classified as minimal (n = 5), mild (n = 5), moderate (n = 4), or severe (n = 2). Monkeys were mated and pregnancy occurred in three monkeys with minimal, three with mild, and one with moderate endometriosis. Laparotomy was performed to stage the disease and to obtain biopsy specimens of implants 1 month post partum or 6 to 8 months after initial operation in nonpregnant monkeys. Monkeys with minimal and mild disease had complete regression of macroscopic disease. Histologic examination of the implants showed only fibrosis and hemosiderin deposits, except in one animal with mild endometriosis containing microscopic foci of endometrial glands. The one monkey with moderate disease had significant, but not complete regression of implants. Endometriosis in nonpregnant monkeys tended to progress. These findings demonstrate that pregnancy does exert a beneficial effect on endometriosis and suggest that pregnancy commonly results in complete resolution of minimal or mild disease.