The heart and its blood vessels start to develop at the end of the third week of conception and the heart begins to beat in the fourth week. The first seven weeks are crucial for its development. Any variation from the normal development pattern during this stage can lead to congenital heart defects.

The circulation of blood in the fetus is different from normal human circulation. In a fetus, the lungs are still not functional and the heart structures are yet to strengthen. The umbilical vein from the placenta carries blood with high oxygen saturation and nutrients. The blood volume in human fetus is estimated to be 10-12% of the body weight, compared with 7-8% in adults. Hemoglobin concentration in the fetal blood is about 50% higher than in mother’s blood. This allows 20-30% increase in the oxygen transport in the fetus.

There are three shunts involved in the fetal circulation. They are –

1. Ductus arteriosus - Carries medium oxygen saturated blood. It allows a low blood flow to the lungs because of a high pulmonary vascular resistance. This ensures protection of lungs against overloading with blood and also allows the right ventricular muscles to strengthen.
   
2. Ductus venosus – Carries high-oxygenated blood from mother to the fetus. It is the blood vessel connecting the umbilical vein to the Inferior vena cava (large vein draining into the heart). It has a sphincter to regulate blood flow.
   
3. Foramen Ovale shunts the oxygenated blood from the right atrium to the left atrium (upper chamber of the heart).

The shunts ensure that the circulating blood in the fetus has high oxygen saturation to meet the demand of rapidly growing fetal tissue.

The blood that enters the fetal circulation divides into two portions under the liver. One portion is transported through the ductus venosus and empties into the Inferior vena cava directly and the other portion enters the liver and is called hepatic circulation and the portal circulation

In the inferior vena cava, the oxygenated blood is mixed with the deoxygenated blood received from the lower parts of the body. This mixed blood enters the right upper chamber of the heart called the right atrium. The right lower chamber of the heart or the right ventricle is usually bypassed.

In adults, the blood that enters the right ventricle is pumped into the lungs to get oxygenated. In the fetus a large portion of blood is shunted into the left atrium (the left upper chamber of the heart) through a foramen ovale. A valve overlying the foramen ovale prevents back flow of blood. The rest of the blood entering the right atrium enters the right ventricle via the pulmonary trunk. As the lungs are collapsed and high resistance is present to the flow, the blood travels through the ductus arteriosus. The lung receives enough blood to sustain its tissues.

The ductus arteriosus shunt connects the pulmonary trunk and the aorta (major artery leaving the heart). Meanwhile, the blood with high oxygen saturation is pumped out to the aorta from the left ventricle. Branches from the ascending aorta supply the brain and the heart muscle. The descending aorta with mixed blood supplies to the lower parts of the body. The rest of the blood goes into the placenta via the umbilical arteries. Here the blood is re-oxygenated and the cycle starts again.