The placenta shelters and nourishes the fetus throughout embryonic development. Through the placenta and the umbilical cord, the baby receives the necessary nutrients, carries out gas exchange and releases all waste products during pregnancy. The origin of these structures is embryonic, although it needs the maternal uterus to create the definitive maternal-fetal structure.
Between the fifth and sixth day after fertilization the embryo is in the blastocyst stage where two clearly differentiated structures can already be distinguished: the inner cell mass and the trophoblast. This last embryonic structure will give rise to the placenta and the umbilical cord.
Specifically, it will be the outer layer of the trophoblast, the syncytiotrophoblast, which will make its way through the endometrium to the internal uterine mucosa, fully connecting the mother's blood vessels with those of the placenta and generating the utero-placental blood circulation.