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OBJECTIVE: The antiprogestin mifepristone is widely used for medical termination of pregnancy (TOP). Previous studies have suggested that the mechanism of mifepristone is based on its action in the endometrium and myometrium. The aim of this study was to evaluate the possible effects of mifepristone on corpus luteum activity. METHODS: This was a prospective, longitudinal controlled study to which 20 patients undergoing medical TOP (study group) and 20 patients with normal ongoing pregnancy (control group) were recruited. Medical TOP was induced with 200 mg of mifepristone followed by 0.8 mg of misoprostol 2 days later. Three-dimensional ultrasound examinations and hormone assays (progesterone, human chorionic gonadotropin, and 17-hydroxyprogesterone) were performed in both groups on the day of, and 2 days after, administration of mifepristone. Total volume (vascularized + non-vascularized) of the dominant (containing corpus luteum) and non-dominant ovary and serum hormone levels were measured. RESULTS: After administration of mifepristone, a decrease in serum progesterone levels was observed with a simultaneous decrease in the non-vascularized volume of the dominant ovary in the study group. No such changes were observed in the control group. CONCLUSIONS: The observations indicate that, in addition to trophoblastic tissue, the corpus luteum is also the target of mifepristone. Copyright (c) 2009 ISUOG. Published by John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]