Progesterone (abbreviated as P4), also known as pregn-4-ene-3,20-dione, is an endogenous steroid and progestogen sex hormone involved in the menstrual cycle, pregnancy, and embryogenesis of humans and other species.rogesterone (abbreviated as P4), also known as pregn-4-ene-3,20-dione, is an endogenous steroid and progestogen sex hormone involved in the menstrual cycle, pregnancy, and embryogenesis of humans and other species.