Final answer:
Mifepristone and ulipristal act as progesterone receptor antagonists or modulators, altering the endometrium, preventing ovulation, and making the cervical mucus less receptive to sperm, thereby serving as effective contraceptive methods.
Step-by-step explanation:
Mifepristone (RU-486) and ulipristal are medications used for medical management in certain gynecological conditions, including emergency contraception and medical abortion. Both drugs act as antagonists or modulators of the progesterone receptor, blocking the effects of progesterone, a key hormone in the maintenance of pregnancy.
These medications lead to changes in the endometrium that prevent implantation of a fertilized egg, or induce shedding if implantation has already occurred. Specifically, mifepristone works by competitively inhibiting the binding of progesterone at its receptor, thus inhibiting progesterone-regulated processes critical for pregnancy continuation. Ulipristal, on the other hand, acts as a selective progesterone receptor modulator (SPRM), altering the endometrium and preventing the progesterone-dependent thickening necessary for embryo implantation.