Before birth, a baby doesn’t breathe through its lungs or eat with its digestive system. Instead, everything it needs comes from the placenta, which connects to the baby by the umbilical cord.

Because of this setup, the fetal circulatory system follows a different route than what we see after birth. Specialized shortcuts, called shunts, help the most oxygen-rich blood reach the organs that need it most, especially the brain and heart, while skipping over organs like the lungs and liver that aren’t fully active yet.

The umbilical cord contains three important blood vessels. One umbilical vein carries freshly oxygenated, nutrient-rich blood from the placenta into the fetus. Two umbilical arteries carry used, deoxygenated blood in the opposite direction, back to the placenta, where waste products can be filtered out by the mother’s system.

Once inside the fetus, the blood takes some clever detours. Normally, blood would travel from the right side of the heart to the lungs to pick up oxygen. But in a fetus, that would be pointless since the lungs are still collapsed and full of fluid. Instead, the blood takes a shortcut through a small opening called the foramen ovale, which lets it move straight from the right atrium to the left atrium.

Some blood still ends up in the pulmonary artery, but that too gets rerouted. A second shunt, the ductus arteriosus, connects the pulmonary artery directly to the aorta. This allows most of that blood to bypass the lungs again and head into the systemic circulation, where it can supply the rest of the body.

There’s also a third shortcut: the ductus venosus. This vessel helps the umbilical vein’s oxygen-rich blood bypass the liver and go straight to the inferior vena cava, which leads to the heart.

After birth, everything changes. As the newborn takes its first breath, the lungs expand, pressure inside the heart shifts, and the shunts gradually close. Over time, the baby’s circulation rearranges itself into the typical adult pattern, with the lungs and liver fully taking over their jobs.