The placenta forms during the embryonic stage. The placenta takes nutrients, oxygen, and water from your blood and passes these along to your baby through the umbilical cord. It also removes the baby's wastes. The placenta will filter out most of the harmful substances that may be present in your body.
The placenta begins to develop upon implantation of the blastocyst into the maternal endometrium, very early on in pregnancy at about week 4. The outer layer of the blastocyst becomes the trophoblast, which forms the outer layer of the placenta.
The development of the placenta begins during implantation of the blastocyst. The 32-64 cell blastocyst contains two distinct differentiated embryonic cell types: the outer trophoblast cells and the inner cell mass. The trophoblast cells form the placenta. The inner cell mass forms the foetus and foetal membranes.
After around 12 weeks of pregnancy, the chorionic villi and uterine tissue become interdigitated with each other and together form a structural and functional unit between developing embryo (foetus) and maternal body called placenta.
Week 4 - implantation In weeks 4 to 5 of early pregnancy, the blastocyst grows and develops within the lining of the womb. The outer cells reach out to form links with the mother's blood supply. After some time, they will form the placenta (afterbirth).
The placenta starts developing very early on in pregnancy at about week 4. Seven or eight days after a sperm fertilizes an egg, a mass of cells — the earliest form of an embryo — implants into the wall of the uterus.
Some of the cells from the placenta develop into an outer layer of membranes (chorion) around the developing blastocyst. Other cells develop into an inner layer of membranes (amnion), which form the amniotic sac. When the sac is formed (by about day 10 to 12), the blastocyst is considered an embryo.
The important role of placenta is to provide nourishment to the developing embryo as well as it plays important role in the exchange of gases between the embryo and mother.