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Blood Circulation in the Fetus and Newborn

How does the fetal circulatory system work?

During pregnancy, the fetal circulatory system works differently than after birth:

Illustration demonstrating fetal circulation

Blood from the mother enters the fetus through the vein in the umbilical cord. It goes to the liver and splits into three branches. The blood then reaches the inferior vena cava, a major vein connected to the heart.

Inside the fetal heart:

Because the placenta does the work of exchanging oxygen (O2) and carbon dioxide (CO2) through the mother's circulation, the fetal lungs are not used for breathing. Instead of blood flowing to the lungs to pick up oxygen and then flowing to the rest of the body, the fetal circulation shunts (bypasses) most of the blood away from the lungs. In the fetus, blood is shunted from the pulmonary artery to the aorta through a connecting blood vessel called the ductus arteriosus.

With the first breaths of air the baby takes at birth, the fetal circulation changes. A larger amount of blood is sent to the lungs to pick up oxygen.

Anatomy of the heart, normal

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