Figure 1. . Schematic diagram of a cross-section through the placenta.

Maternal blood arrives to the placenta through the spiral arteries to perfuse the intervillous space and drains through the maternal veins. Nutrient and waste exchange (white double-headed arrow) occurs across the syncytiotrophoblast layer from the maternal intervillous blood pool to the fetal capillaries within the chorionic villi. Nutrient-rich blood is transported to the fetus via the umbilical vein, and deoxygenated blood returns via the two umbilical arteries.