From Creatures Wikia
Oestrogen (also spelled Estrogen) is a chemical which makes female creatures receptive to sperm, allowing them to get pregnant (sometimes in combination with other chemicals.) It is normally produced periodically as part of a cycle, though creatures can be genetically engineered to produce it constantly or in response to certain stimuli. It can also be increased by an agent or COB or via directly injecting it into a creature's bloodstream. In C3 and DS it has the chemical number 46, in C2 105, and in C1 63.