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:
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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.
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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.
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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:
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Oxygen and nutrients from the
mother's blood are transferred across the placenta to the fetus through the
umbilical cord.
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This enriched blood flows
through the umbilical vein toward the baby’s liver. There it moves through a shunt
called the ductus venosus.
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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:
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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.
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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).
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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.
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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.