Implantation is the process by which embryo is fixed and embedded in endometrium of uterus. Decidual reaction-under influence of hcG secreted by syncytiotrophoblast, secretary phase changes in endometrium are intensified. Decidua is divided in three parts after implantation: 1. Decidua basalis, 2. Decidua parietalis, and 3. Decidua capsularis. Chorionic villi develop in three stages: 1. Primary villi, 2. Secondary villi, and 3. Tertiary villi. Placental barrier or membrane separates maternal blood from foetal blood. Across this membrane various substances move to and fro between mother and foetus.