To understand the placental diseases, normal placenta should be learned well.
Placenta is composed of chorionic villi which provide a large contact area between the maternal and fetal circulations. In the mature placenta maternal blood enters the intervillous space by endometrial arteries and circulates around the villi to allow the gas and nutrient exchange. Deoxygenated blood leaves the intervillous space through endometrial veins.
Deoxygenated fetal blood comes to the chorionic villus by umbilical artery. Blood is oxygenated in the chorionic villi which is in a capillary structure. The oxygen and nutrient diffusion occurs through the villous capillary endothelial cells and thinned-out syncytiotrophoblast and cytotrophoblast. Then blood flows back to the fetus by a single umbilical vein.
Under normal state there is little or no mixing between maternal and fetal blood, though enough free fetal DNA reaches to the maternal blood to permit prenatal genetic testing.