At the implantation location, the maternal endometrium is changed by the decidual reaction (epithelial transformation of the fibroblasts of the uterine stroma, in that lipids and glycogen accumulate) and is called the decidua.
The decidua consists of various parts, depending on its relationship with the embryo:
- Decidua basalis, where the implantation takes place and the basal plate is formed. This can be subdivided into a zona compacta and a zona spongiosa (where the detachment of the placenta takes place following birth).
- Decidua capsularis, lies like a capsule around the chorion
- Decidua parietalis, on the opposite uterus wall
At around the 4th month, the fetus is so large that the decidua capsularis comes into contact with the decidua parietalis. The merging of these two deciduae causes the uterine cavity to obliterate.