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Fetal blood, or fetal hemoglobin, is the blood that circulates in a fetus from the time the child is about two months into gestation. This same type of blood remains in a newborn baby until he or she is about six months old. It has certain properties, tailored to suit the specific needs of a fetus and an infant. Some blood disorders, like sickle-cell anemia, can be affected by this special form of human hemoglobin.
Since the fetus gets all of its oxygen and nutrients from its mother's blood for the duration of the pregnancy, this gestational blood is more efficient at processing oxygen than the blood humans make later in life. It contains about 50% more hemoglobin, which is a protein in red blood cells that circulates oxygen. That hemoglobin is able to hold 20 to 30% more oxygen than normal blood. The major vein of the umbilical cord, called the umbilical vein, gets oxygen-rich blood from the mother, through the placenta, and to the fetus.
Once the baby is born, fetal blood decreases to up to 50%. It continues to decrease until about six months of age, when it is completely replaced by typical human blood. Occasionally, this process fails and fetal blood continues to be the main type of blood circulated throughout the system well into childhood and adulthood. This is a condition known as hereditary persistence of fetal hemoglobin (HPFH). It has no known symptoms and is usually discovered only when testing for other blood diseases. The condition is particularly prevalent in people of African and Greek heritages.
Another anomaly that can happen when fetal blood changes is an indicator of sickle-cell disease, sometimes called sickle cell anemia. Instead of producing adult, or typical, blood at six months like the majority of babies do, those with sickle-cell disease make hemoglobin S — a form of hemoglobin that causes red blood cells to be sickle-shaped instead of round. This abnormality usually results in periods of intense pain. If the person suffering from sickle-cell still has predominately fetal blood instead of hemoglobin S, the pain is less intense and the episodes are less frequent. Therefore, a synthetic drug called hydroxyurea is often used to help produce more fetal blood to treat these episodes.
Fetal blood can be sampled, in utero, for various blood diseases. The process is considered an outpatient surgery. Using an ultrasound machine, a doctor inserts a needle through the mother's torso and into the umbilical cord. The blood is withdrawn through the needle and then tested for anemia and other problems.