A fetus (baby) is fed, or nourished, by the mother through the
placenta, which is attached to the umbilical cord. In the placenta, the
mother's blood and the fetal blood both flow through vessels that are very
close together. But the mother's blood does not mix with the fetal blood. When
the mother's blood is close to the fetal blood, oxygen and nutrients move from
the mother's blood into the fetal blood.
As the blood flows through the fetus, it picks up waste products and
returns to the mother through the umbilical cord. The blood (with waste
products from the fetus) goes through the mother's lungs and liver, where waste
products are removed.
Since oxygen is supplied by the mother, the fetus does not use lungs
to breathe. Only a small amount of blood flows to the fetus's lungs. After
birth, blood must flow to the baby's lungs. Before birth, the mother's liver
removes waste products for the fetus, so less blood flows through the fetus's
Blood flows around the fetus's lungs and liver by going through an
opening in the heart and through two extra blood vessels. This opening (called
the foramen ovale) and the extra blood vessels (called the ductus arteriosus
and the ductus venosus) normally close after birth.
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Foramen ovale. The foramen
ovale is an opening in the wall that separates the upper right and left heart
chambers (atria). This opening allows blood to flow to the left side of the
heart without going to the lungs. Before birth, the foramen ovale is kept open
by the pressure of blood that passes through it. When the baby takes the first
breath, blood begins to flow through the lungs, and the foramen ovale
Ductus arteriosus. The ductus
arteriosus is a blood vessel that allows fetal blood to bypass the lungs. This
vessel usually partially closes 10 to 15 hours after birth and completely seals
closed by 10 to 21 days after birth. The closure of the ductus arteriosus after
birth allows blood flow to reach the lungs only through the pulmonary
Ductus venosus. The ductus venosus
is a vessel that allows blood to bypass the fetus's liver. It carries blood
with oxygen and nutrients from the umbilical cord straight to the right side of
the fetus's heart. The ductus venosus closes shortly after birth, when the
umbilical cord is cut and blood flowing between the mother and fetus
John Pope, MD - Pediatrics & Larry A. Latson, MD - Pediatric Cardiology
How this information was developed to help you make better health decisions.