Progesterone is a chemical used in the reproductive system in the Creatures series.
It is created by female creatures when they are pregnant, and reacts with oestrogen to make them temporarily infertile. Over the course of a pregnancy, progesterone builds up in a creature, until a certain point is reached. When the progesterone is high enough, this triggers the female to lay her egg. After this occurs, the progesterone will decay away until the normal fertility cycle resumes.