There are lots of differences in circulation between the adults and a fetus in utero (womb). To understand this, first you should know the basics of circulation in adults.

Adult circulation:

Haematopoiesis is the process by which the blood cells are formed. In adults the blood cells are formed in the bone marrow of bones such as sternum, pelvis, vertebra and cranial bones. Blood cells together with plasma forms the blood. Blood has 45% blood cells and 55% plasma. Plasma is nothing but straw colored fluid containing water, glucose and proteins.

Blood cells include

1. Red blood cells (RBC) /Erythrocytes
2. White blood cells (WBC) /Leucocytes and
3. Platelets / Thrombocytes.

The major function of RBC is to carry oxygen from the lungs to the tissues and carbon-di-oxide from tissues to the lungs. This major function of RBC is done by hemoglobin (alpha 2 beta 2 chain-adult type) which is made of heme and globin chain. Each hemoglobin molecule can bind to four molecules of oxygen.

Once blood is formed, it is a deoxygenated blood. This blood is carried to the right atrium of the heart by the veins (Superior and inferior vena cava). SVC brings the venous blood from above the level of heart while IVC brings it from below the level of the heart. From right atrium the blood flows passively into the right ventricle through a valve in between the two which is called the mitral valve. From the right ventricle the blood is taken to the lungs by the pulmonary arteries where it undergoes a process of oxygenation. Now the blood is called oxygenated blood. The oxygenated blood from the lungs is taken to the left atrium by the four pulmonary veins. The blood from the left atrium of the heart is pumped into the left ventricle. Once the blood is pushed out from the left ventricle, the blood is carried by the arteries to different parts of the body. Thus the tissues take oxygen from the blood and direct the blood towards the vein and again the cycle continues.

If you notice the oxygen to the adults is derived from the surroundings.

Fetal circulation:

Haematopoiesis takes place in the yolk sac by two weeks of fetal life. It is continued by the yolk sac upto tenth week. Thereafter the process is taken over by the liver. This is the physiological reason for the liver to be enlarged in a newborn. In infants the process is carried over by the bone marrow of long bones such as the femur and tibia. Then it resembles an adult type.

Fetal Hemoglobin (Alpha 2 Gamma 2) has more affinity for the oxygen than the adult type. The life span of fetal RBC is 80 days which is less in comparison with adult RBC (120 days).

Regarding circulation in the fetus, lung is not the site of oxygenation. It is the placenta. The oxygen supply to the fetus has no communication with the exterior world. 

The oxygenated blood is carried by the umbilical vein from the placenta. Umbilical vein in the liver gives rise to right and left branches. The blood from the right branch is directly taken to the IVC whereas the blood from the left branch mixes with the deoxygenated blood from the portal vein and then reaches the IVC through the ductus venosus and thence to the right atrium of the heart.

From the right atrium about 75 % of the blood is directed to the left atrium through the foramen ovale which is a communication between the two atria thus bypassing the right ventricle. This blood is then pumped into the left ventricle and then to the aorta.

Remaining 25 % of the blood is pumped into the right ventricle. But the pressure in the pulmonary artery of the fetus is so high that the blood cannot be pumped into it. Instead the blood is directed into the aorta through the ductus arteriosus which is the communication between the pulmonary artery and the aorta. The blood from the aorta is then taken to the different parts of the body. About 40 % of the blood from the aorta is taken to the placenta by the right and left umbilical arteries. Thus there is no role of lungs in the fetus.

Immediately after birth, some changes occur in the circulation. There are two reasons for this.

1.  Initiation of respiration
2. The placenta is cut. So there is no more placental blood flow.

Changes of fetal circulation at birth:

1.  Closure of the umbilical arteries.
2.  Closure of the umbilical vein: After its anatomical obliteration the umbilical vein becomes the ligamentum teres and ductus venosus becomes    ligamentum venosum.  
3.  Closure of the ductus arteriosus: After its anatomical obliteration it becomes ligamentum arteriosum.
4.  Closure of the foramen ovale.

By these changes adult circulation is established. If there is a defect in the closure of ductus arteriosus or foramen ovale then the child develops congenital heart diseases.

Images: From Wikimedia commons.