The circulatory system, including the heart, is one of the first systems that develops after conception. A human embryo has a basic, functioning circulatory system and heart by the end of the first month of life. This circulatory system is vital for sustaining life because it nourishes tissues with oxygen-rich blood and removes waste products from the body.

Throughout gestation, the human heart takes on various forms. In its earliest phase, the heart is simply a long tube, much like a fish heart. As it rapidly grows, it folds back over on itself, forming the familiar heart shape. At this point, the heart is proportionally much larger than it will be at any other time of life, occupying much of the fetal midsection. 

Throughout the second month of gestation, the internal structures of the heart are rapidly developing. The two atria are formed first, while there still remains just one large ventricle. At this point, the atria are only partially separate, joined by a hole called the foramen ovale.

There is initially only one artery arising from the heart but this divides and rotates so that the artery supplying the lungs comes from the right ventricle and the one supplying the body arises from the left. Other arteries coming off the early fetal, common artery or truncus develop into vessels going to the head and arms. Another (ductus arteriosus) remains to connect the artery leading to the lungs and the aorta (main artery to the body). Other vessels simply disappear. The ventricles are slightly slower to separate, and their separation marks the end of the development of the fetal heart.

By the end of the second month, the fetal heart has all four chambers and four valves. It has already begun to beat, at about 148 beats per minute, and supply the developing fetus with blood. By this time, the fetal lungs have formed but are not yet functioning. Instead, the fetus receives oxygen-rich blood from its mother through a vein in the umbilical cord attached to the placenta.

This oxygen-rich maternal blood enters the right atrium of the fetal heart through the inferior vena cava, where it mixes with oxygen-poor fetal blood returning from the upper body through the superior vena cava. Most of the blood from the inferior vena cava then travels to the left atrium through the foramen ovale – a normal prenatal hole in the wall between the upper chambers of the heart (atria). It then goes to the left atrium and from there to the main pumping chamber of the heart, the left ventricle. This blood, which is more oxygen rich, is then pumped out the aorta to those organs requiring the highest level of oxygen in the fetus, the brain.  Most of the more oxygen poor blood from the upper portion of the body coming back in the superior vena cava is directed to the right ventricle and from there into the pulmonary artery. It then is directed through a prenatal passageway called the ductus arteriosus and out through the aorta to nourish the lower body and back to the placenta in the two umbilical arteries. This combination of the foramen ovale and ductus arteriosus blood flow is responsible for bypassing the fetal lungs, which are not yet functioning.

Waste products from the developing fetus are carried in the blood back to the placenta via two arteries in the umbilical cord. These waste products diffuse into the mother’s blood and are carried away and excreted by the maternal circulation. Since the fetal lungs are basically collapsed during all of pregnancy, the mother and the transfer of oxygen through the placenta do all the work of breathing and getting oxygen to the fetus.