The placenta is a temporary organ required for the development of the embryo and fetus. It allows for the exchange of nutrients and oxygen from mother to fetus, and the transfer of fetal waste products back to the mother for disposal by her organs. There is no mixing of maternal and fetal blood in the placenta. In this section, you will learn about the circulation of blood through the fetus and placenta, and how the fetus is nourished and gets rid of its waste. The blood system in the placenta also limits what chemicals and infectious agents in the mother's body can cross the placenta to endanger the life of the fetus.
As we mentioned in the previous section, the placenta starts to develop from the trophoblast, after the embryo implants in the uterus. It is fully developed within two months, and it continues growing until labour begins. The placenta weighs about 600 gm on average at delivery.