There are two vascular components in every placenta; the maternal circulation and the fetal circulation. The maternal component is fully formed roughly by the 24th to 26th week of pregnancy but the fetal circulation continues to grow until 38-40 weeks. The healthy development of the fetus depends on a healthy placenta (afterbirth). The placenta plays the single most vital role in the development of a healthy fetus and is formed by the connection of fetal and maternal vessels. The fetal vessels (thousands of them) are contained within the chorionic villi (microscopic projections of fetal tissue), which are completely submerged in maternal blood. Oxygen and nutrients are therefore exchanged between maternal and fetal blood.