Can you guess why the heart of a fetus (that's the name for a baby before it is born) is different from that of a baby (and an adult)?

Here's a hint: does the fetus get oxygen from its own lungs? Is the fetus breathing?

NO! The fetus gets oxygen from the mother, from the mother's placenta to be exact. Therefore, the heart of a fetus does not need to pump blood to its lungs. In fact, less than 10% of the blood of a fetus passes through the lungs since the lungs are collapsed and filled with fluid.

Because of this, the fetal heart is different from the heart of a baby or an adult. These differences are what allow most fetuses to survive severe heart defects while inside the mother.

1) There is an extra blood vessel in the fetus that allows the blood to BYPASS the lungs. This is called the DUCTUS ARTERIOSIS. This blood vessel connects the pulmonary artery (the highway leading to the lungs) and the aorta (the highway leading to the body). It is like a shortcut. After a baby is born, the ductus arteriosus is supposed to close. If it remains open, it is called a PATENT (meaning still open) DUCTUS ARTERIOSIS - or PDA (no - not that type of PDA!). Many babies who are born prematurely, meaning earlier than expected, will have a PDA. The PDA can cause problems in babies after they are born because too much blood will end up going to the lungs and the lungs can get flooded.  In certain types of severe CHD, the PDA is very important to keep the baby alive while awaiting surgery.

2) The wall between the two upper chambers of the heart - the right and left atrium - has a hole in it. This is called the FORAMEN (fancy word for hole) OVALE (fancy word for oval shape). This hole allows blood coming into the heart from the placenta to go directly to the left side of the heart to bring oxygen to the heart muscle and brain. The heart and brain are greedy like that - they want to get at all the yummy oxygen before the rest of the body. And they should - because they are very important. After a baby is born, this hole is closed by a flap. If it remains open, it is called a PATENT (fancy word for open) FORAMEN OVALE or PFO.