The placenta is formed from the chorionic villi, small fingerlike projections that cover the outer cells of the blastocyst. After implantation of the fertilized ovum, the chorionic villi burrow into the lining of the |uterus| seeking nourishment; those which have penetrated deepest erode some of the small uterine ~blood~ vessels and become bathed in the mother's ~blood~. At this point the burrowing stops and the villi start to multiply and form branches. It is these villi which form the basis of the placenta.
The placenta is responsible for the transfer of nourishment from the mother to the |fetus|, and of the waste products the |fetus| produces to the mother so that they can be excreted. Two layers of cells keep the fetal circulation in the placenta separate from the maternal ~blood~. Through these cells the vital exchange function of the placenta takes place. Carbon dioxide, waste products and ~hormones~ pass from the |fetus| to the mother. Oxygen, nutrients and ~hormones~ are transferred in the opposite direction. The placenta also acts as a barrier to protect the |fetus| against potentially harmful substances. Many medications, however, can cross the placenta and harm the |fetus|. Certain medications, as well as drugs and |alcohol|, should not be taken while pregnant for this reason.
