The blood that flows through the fetus is actually more complicated than after the baby is born (normal heart). This is because the mother (the placenta) is doing the work that the baby’s lungs will do after birth. The placenta accepts the bluest blood (blood without oxygen) from the fetus through blood vessels that leave the fetus through the umbilical cord (umbilical arteries, there are two of them). When blood goes through the placenta it picks up oxygen and becomes red. The red blood then returns to the fetus via the third vessel in the umbilical cord (umbilical vein). The red blood that enters the fetus passes through the fetal liver and enters the right side of the heart. The red blood goes through one of the two extra connections in the fetal heart that will close after the baby is born. The hole between the top two heart chambers (right and left atrium) is called a patent foramen ovale (PFO). This hole allows the reddest blood to go from the right atrium to left atrium and then to the left ventricle and out the aorta. As a result the blood with the most oxygen gets to the brain. Blood coming back from the fetus’s body also enters the right atrium, but the fetus is able to send this blue blood from the right atrium to the right ventricle (the chamber that normally pumps blood to the lungs). Most of the blood that leaves the right ventricle in the fetus bypasses the lungs through the second of the two extra fetal connections known as the ductus arteriosus. The ductus arteriosus sends the bluer blood to the organs in the lower half of the fetal body. This also allows for the bluest blood to leave the fetus through the umbilical arteries and get back to the placenta to pick up oxygen. Since the patent foraman ovale and ductus arteriosus are normal findings in the fetus, it is impossible to predict whether or not these connections will close normally after birth in a normal fetal heart. These two bypass pathways in the fetal circulation make it possible for most fetuses to survive pregnancy even when there are complex heart problems and not be affected until after birth when these pathways begin to close. source
