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 liver.

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 closes.
• 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 artery.
• 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 stops.