I am not expert but I think OP means different hormones would start to get produced and others would become suppressed to support the creation of a placenta, to support the the lining of the uterus and to cease ovulation and menstrual cycle as well as to start supporting the growing embryo in terms of nutrients by means of generating and connecting blood vessels.

I read somewhere that pregnancy (unlike what is normally described) is a tug of war between the embryo which is the leach if you will on the mother which is the host. The embryo basically try consumes the host and so long as everybody is doing what they're supposed to, all the mechanisms end up keeping that war at bay with both participants making it alive at the end. If some mechanisms (and signals) were to misbehave one of the two would cease to exist.