The fetal circulation works differently from that of an adult, mainly because the lungs are not in use in fetal circulation. Lungs are bypassed in fetal circulation. A hole in the septum called the foramen ovale, allows blood from the right atrium to enter the left atrium during fetal development. This opening allows blood to bypass the nonfunctional fetal lungs while the fetus obtains its oxygen and nutrients from the mother through the placenta and the umbilical cord. A layer of tissue called the septum primum acts as a valve over the foramen ovale during fetal development. After birth, the pressure in the right side of the heart drops as the lungs open and begin working, causing the foramen ovale to close entirely.

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