How does the uterus prepare itself for the implantation of the embryo, & what happens when the implantation doesn't take place?

How does the uterus prepare itself for the implantation of the embryo,

1.	How does the uterus prepare itself for the implantation of the embryo, & what happens when the implantation doesn't take place?
Answer» How does the uterus prepare itself for the implantation of the embryo, & what happens when the implantation doesn't take place?