Progesterone means 'for pregnancy'. Why is it called so?

The corpus luteum secretes large amounts of progesterone which is essential for maintenance of the endometrium. Such an endometrium is necessary for implantation of the fertilised ovum and other events of pregnancy. Hence, it is called the pregnancy hormone.

High levels of progesteron maintains endometrium. Waning of progesteron causes menstruation.

Progesteron prepares mammary glands for the lactation during pregnancy. Ejection of milk is due to oxytocin.

A high level of progesterone ensures that the uterine myometrium is relaxed and that the cervix is tightly closed.