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:
The fetus is connected by the umbilical cord to the placenta, the organ that develops
and implants in the mother's uterus during pregnancy.
Through the blood vessels in the umbilical cord, the fetus receives all the necessary
nutrition, oxygen, and life support from the mother through the placenta.
Waste products and carbon dioxide from the fetus are sent back through the umbilical
cord and placenta to the mother's circulation to be eliminated.
The fetal circulatory system uses two right to left shunts, which are small passages
that direct blood that needs to be oxygenated. The purpose of these shunts is to bypass
certain body parts – in particular, the lungs and liver – that are not fully developed while
the fetus is still in the womb. The shunt that bypass the lungs is called the foramen
ovale. It moves blood from the right atrium of the heart to the left atrium. The ductus
arteriosus moves blood from the pulmonary artery to the aorta.
Oxygen and nutrients from the mother's blood are transferred across the placenta to
the fetus. The enriched blood flows through the umbilical cord to the liver and splits
into three branches. The blood then reaches the inferior vena cava, a major vein connected
to the heart. Most of this blood is sent through the ductus venosus. It is also a
shunt that passes highly oxygenated blood through the liver to the inferior vena cava
and then to the right atrium of the heart. A small amount of this blood goes directly
to the liver to give it the oxygen and nutrients it needs.
Waste products from the fetal blood are transferred back across the placenta to the
Inside the fetal heart:
Blood enters the right atrium, the chamber on the upper right side of the heart. When
the blood enters the right atrium, most of it flows through the foramen ovale into
the left atrium.
Blood then passes into the left ventricle (lower chamber of the heart) and then to
the aorta (the large artery coming from the heart).
From the aorta, blood is sent to the heart muscle itself in addition to the brain. After
circulating there, the blood returns to the right atrium of the heart through the
superior vena cava. About two thirds of the blood will pass through the foramen ovale
as described above, but the remaining one third will pass into the right ventricle,
toward the lungs.
In the fetus, the placenta does the work of breathing instead of the lungs. As a result,
only a small amount of the blood continues on to the lungs. Most of this blood is
bypassed or shunted away from the lungs through the ductus arteriosus to the aorta.
Most of the circulation to the lower body is supplied by blood passing through the
This blood then enters the umbilical arteries and flows into the placenta. Carbon
dioxide and waste products are released into the mother's circulatory system, and
oxygen and nutrients from the mother's blood are released into the fetus' blood.
At birth, the umbilical cord is clamped and the baby no longer receives oxygen and
nutrients from the mother. With the first breaths of life, the lungs begin to expand,
the ductus arteriosis and the foramen ovale both close.