How does the fetal circulatory system work?

During pregnancy, the unborn baby (fetus) depends on its mother for nourishment and oxygen. Since the fetus doesn’t breathe air, their blood circulates differently than it does after birth:

• The placenta is the organ that develops and implants in the mother's womb (uterus) during pregnancy. The unborn baby is connected to the placenta by the umbilical cord.

• All the necessary nutrition, oxygen, and life support from the mother’s blood goes through the placenta and to the baby through blood vessels in the umbilical cord.

• Waste products and carbon dioxide from the baby are sent back through the umbilical cord blood vessels and placenta to the mother's circulation to be eliminated.

While still in the uterus, the baby's lungs aren't being used. The baby’s liver isn't fully developed. Circulating blood bypasses the lungs and liver by flowing in different pathways and through special openings called shunts.

Blood flow in the unborn baby follows this pathway:

• Oxygen and nutrients from the mother's blood are transferred across the placenta to the fetus through the umbilical cord.

• This enriched blood flows through the umbilical vein toward the baby’s liver. There it moves through a shunt called the ductus venosus.

• This allows some of the blood to go to the liver. But most of this highly oxygenated blood flows to a large vessel called the inferior vena cava and then into the right atrium of the heart.

Here is what happens inside the fetal heart:

• When oxygenated blood from the mother enters the right side of the heart, it flows into the upper chamber (the right atrium). Most of the blood flows across to the left atrium through a shunt called the foramen ovale.

• From the left atrium, blood moves down into the lower chamber of the heart (the left ventricle). It's then pumped into the first part of the large artery coming from the heart (the ascending aorta).

• From the aorta, the oxygen-rich blood is sent to the brain and to the heart muscle itself. Blood is also sent to the lower body.

• Blood returning to the heart from the fetal body contains carbon dioxide and waste products as it enters the right atrium. It flows down into the right ventricle, where it normally would be sent to the lungs to be oxygenated. Instead, it bypasses the lungs and flows through the ductus arteriosus into the descending aorta, which connects to the umbilical arteries. From there, blood flows back into the placenta. There the carbon dioxide and waste products are released into the mother's circulatory system. Oxygen and nutrients from the mother's blood are transferred across the placenta. Then the cycle starts again.

At birth, major changes take place. The umbilical cord is clamped, and the baby no longer receives oxygen and nutrients from the mother. With the first breaths of air, the lungs start to expand, and the ductus arteriosus and the foramen ovale both close over the first minutes and days of life. The baby's circulation and blood flow through the heart now function like an adult's.