antiprogestin

noun

an·​ti·​pro·​ges·​tin -prō-ˈjest-ən 

variants also anti-progestin or antiprogestogen or anti-progestogen

: a substance (such as mifepristone) that inhibits the action of progesterone in the body

RU-486 is an antiprogestin that is thought to compete with progesterone at the receptor site.

—Charles Morgan, Nature