During pregnancy, the fetal circulatory system works differently than after birth:
* The fetus is connected by the umbilical cord to the placenta, the organ that develops and implants in the mother's uterus during pregnancy.
* Through the blood vessels in the umbilical cord, the fetus receives all the necessary nutrition, oxygen and life support from the mother through the placenta.
* Waste products and carbon dioxide from the fetus are sent back through the umbilical cord and placenta to the mother's circulation to be eliminated.
* Blood from the mother enters the fetus through the vein in the umbilical cord. It goes to the liver and splits into three branches. The blood then reaches the inferior vena cava, a major vein connected to the heart.
Inside the fetal heart:
* Blood enters the right atrium, the chamber on the upper right side of the heart. Most of the blood flows to the left side through a special fetal opening between the left and right atria, called the foramen ovale.
* Blood then passes into the left ventricle (lower chamber of the heart) and then to the aorta, (the large artery coming from the heart).
* From the aorta, blood is sent to the heart muscle itself in addition to the brain and arms. After circulating there, the blood returns to the right atrium of the heart through the superior vena cava.
* About one-third of the blood entering the right atrium does not flow through the foramen ovale, but, instead, stays in the right side of the heart. This blood enters the right ventricle from the right atrium, then exits the right ventricle to end up in the pulmonary artery. From there, some of the blood will travel to the lungs. The majority of the blood in the pulmonary artery, however, enters the descending aorta through a special artery called the patent ductus arteriosus (PDA). It then travels through smaller vessels to reach back into the placenta.