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Mifepristone is a progesterone receptor antagonist widely used in obstetrics. The aim of the study was to focus on free corticotrophin-releasing hormone (CRH) and also describe modulation of adrenal and placental steroid hormone concentrations induced by mifepristone. Methods: Twenty-six women were enrolled in the study. They received mifepris-tone for termination of pregnancy. Maternal blood samples were retrieved before administration of mifepristone (600 mg) and 48 h after, just before induction of labor. Bound and free CRH levels were determined in maternal blood concomitantly with Cortisol, estriol, progesterone and SDHEA levels. Also paired fetal cord blood samples were collected. Results: Maternal plasmatic CRH level did not change after mifepristone absorption but free CRH increased significantly (0.500 ± 0.326 vs. 0.388 ± 0.303 ng/ml, p = 0.040). A significant decrease of progesterone was observed (83.6 ± 49.3 vs. 95.6 ± 54.9 ng/ml, p = 0.001) with a lower progester-one/estriol ratio (26.9 ± 15.7 vs. 40.7 ± 31.1, p = 0.004). There was a strong association between maternal and fetal free CRH (r² = 0.675, p = 0.001), Cortisol (r² = 0.570, p = 0.019), and positive but modest correlation for progesterone (r² = 0.341, p = 0.046) and estriol (r² = 0.379, p = 0.025) levels. Conclusion: Mifepristone has an effect on free CRH level and changes the estriol-progesterone balance. [ABSTRACT FROM AUTHOR]