The fetus does not use its own lungs until birth, so its circulatory system is different from that of a newborn baby.
The baby growing inside of the mother’s uterus (the womb) is called a fetus. The growing fetus is fully dependent on a special organ called the placenta for nourishment. One side of the placenta is attached to the uterus, and the other side is attached to a liquid-filled sac that holds the fetus. A special cord called the umbilical cord links the placenta to the fetus. The mother’s blood flows through a thin layer of cells in the wall of the uterus, giving the fetus food and oxygen while removing any wastes like carbon dioxide. There is actually no direct contact between the circulatory systems of the mother and fetus.
The fetus does not use its own lungs until birth, so its circulatory system is different from that of a newborn baby. Before birth, the fetal heart does not have to pump blood to the lungs to pick up oxygen. In other words, the fetal heart does not need a separate pulmonary artery and aorta. In the fetal heart, these two blood vessels are connected by a blood vessel called the ductus arteriosus. After birth, the ductus closes and a separate left pulmonary artery and aorta form.
The fetal heart also has an opening between the upper chambers (the right and left atria) called the foramen ovale. It lets blood flow directly from the right atrium to the left atrium during fetal development, but closes after birth. So the ductus arteriosus and the foramen ovale are part of the fetal circulatory system before birth but disappear soon after birth.
In most babies, these blood-flow routes naturally close up shortly after birth, when the baby’s lungs and cardiovascular system take over. Because the fetal heart has a circulatory system different from the one the baby uses after birth, it may be days or weeks before some congenital heart defects are found.