Pregnancy is established approximately one week after fertilization. The blastocyst embeds itself in the thickened wall of the uterus through a process called implantation. Implantation is followed by the appearance of finger-like projections on the trophoblast called chorionic villi which are surrounded by the tissue and maternal blood. Interdigitation of the chorionic villi and uterine tissue with each other leads to the formation of a structural and functional unit called the placenta between the developing embryo (foetus) and maternal body.
In humans, by the fourth month of pregnancy, the placenta has two components i.e., chorion frondosum (foetal component) and decidua basalis (maternal component). By the fifth month, decidua forms into the decidual septa. A part of it interacts with the trophoblast and develops into a structure called deciduas basalis, while the remainder is called decicua parietalis or vera. The yolk sac works as a source of nourishment and is the first visible structure in the gestational sac which covers the foetus.
