The implantation of the blastocyst commences the development of placenta. The 32-64 cell blastocyst has – the inner cell mass and the outer trophoblast cells. These trophoblast cells form placenta while the inner cell mass goes on to form the foetus and the various foetal membranes. The placenta is fully formed by 18-20 weeks, however, it continues to grow all through the pregnancy.
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