Fetal Anemia
Fetal anemia occurs when the number of red blood cells (RBCs) falls below normal levels before delivery. RBCs carry oxygen to the fetal organs. If the RBC count is too low, the fetus may not receive enough oxygen. Severe fetal anemia is rare. When it does occur, there is an increased risk of serious problems like heart failure (when the heart cannot keep up with pumping blood around the body) and abnormal fluid buildup. If there is a risk of fetal anemia, testing is done throughout pregnancy to monitor the fetus. In some cases, treatments can be given to prevent long-term health problems.
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A common cause of fetal anemia in the United States is RBC alloimmunization. This condition occurs when the fetus has a protein on their RBCs that the mother does not have. The fetus inherits this protein from the father. If fetal RBCs cross into the maternal bloodstream, the mother’s immune system treats these cells as foreign. It then makes antibodies against them. These antibodies remain in the mother’s bloodstream even after pregnancy. In a later pregnancy, if the fetus has the same protein, the mother’s antibodies can cross the placenta and attack the fetal RBCs. This can lead to a condition known as hemolytic disease of the fetus and newborn (HDFN).
Fetal anemia can also be caused by parvovirus infection. Parvovirus is a common childhood infection that causes a rash on the face, torso, arms, and legs. In a fetus, the virus can attack developing RBCs in the bone marrow. The result can be severe anemia, especially if the infection occurs before 20 weeks of pregnancy. Other infections, such as cytomegalovirus (CMV), coxsackie virus, and syphilis, may also cause fetal anemia.
Fetal anemia can also occur with certain genetic and metabolic disorders. Some twin pregnancies that share a placenta are also at risk of fetal anemia.
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To check for anemia, ultrasound may be used to measure the blood flow in an artery in the fetus’s brain. This artery is called the middle cerebral artery (MCA). One of the fetal responses to anemia is to increase blood flow to the brain. This test is used to screen for fetal anemia in several conditions, including RBC alloimmunization and parvovirus.
A diagnosis of fetal anemia can be made only by sampling blood from the umbilical cord. This procedure, called cordocentesis, is performed at specialized centers. There is a small risk of pregnancy loss and early delivery with this procedure. Stillbirth is reported in 1% to 2% of pregnancies following cordocentesis. This can be related to the procedure or to the underlying fetal anemia.
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Monitoring varies depending on the suspected cause of the disease. If RBC alloimmunization is suspected, a key step is to test the father’s blood. This is done to find out if the father’s RBCs carry a protein that the mother does not have. If that is not possible, amniocentesis followed by genetic testing is done to see if the fetus has the RBC protein. In some cases, a cell-free DNA test can be used instead of amniocentesis. This test uses a sample of the mother’s blood.
If a pregnant person has antibodies against the fetus’s blood, antibody levels are tested throughout pregnancy. If antibody levels are too high, more tests may be needed. These include ultrasound exams to measure MCA blood flow or a cordocentesis to directly test for fetal anemia.
In cases where fetal anemia happened in a prior pregnancy due to maternal antibodies, antibody levels are not helpful in the current pregnancy. Ultrasound monitoring or direct testing of fetal RBC levels may be done to check for fetal anemia.
If a pregnant person thinks they’ve been exposed to parvovirus, a test can show if they have had it in the past and are now immune. If not, and there is a current infection, monitoring with ultrasound may be needed throughout the pregnancy to check for signs of fetal anemia.
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Treatment varies depending on the suspected cause of the disease. If RBC alloimmunization is suspected, a key step is finding out the blood type of the father to determine whether the fetus is at risk. If that is not possible, the fetal blood type can be determined using amniocentesis or, in some cases, by testing a sample of your blood using cell-free DNA technology. If you have antibodies against the fetus’s blood, you will be tested several times during pregnancy to measure your antibody levels. If antibody levels are too high, you may have additional tests such as extra ultrasound exams to evaluate blood flow in the fetal brain or a fetal blood test to check the fetus’s health.
If you’ve had a prior pregnancy complicated by fetal anemia due to maternal antibodies, antibody levels are not helpful in the current pregnancy. Ultrasound or direct testing of fetal RBC levels may be done to find the cause of the fetal anemia.
If you were exposed to parvovirus, your healthcare professional will test to see if you had parvovirus infection in the past and are now immune. But if you haven’t had it in the past, and a current infection is confirmed, you may need close monitoring with ultrasound during your pregnancy to check for signs of fetal anemia.
Quick Facts
Fetal anemia occurs when the number of red blood cells (RBCs) falls below normal levels in a developing fetus. Severe fetal anemia can increase the risk of serious health problems. Testing can be done to monitor fetal health. In some cases, treatments can be given to prevent long-term health problems.
The most common cause of fetal anemia is incompatibility between the mother’s and fetus’s red blood cells. Other causes include infections and genetic disorders.
Fetal anemia is diagnosed by taking a blood sample from the umbilical cord. Ultrasound may be used to confirm the diagnosis and monitor the fetus’s health throughout the pregnancy.
Severe cases of fetal anemia can be treated with a blood transfusion given to the baby before birth (intrauterine transfusion), performed by specialists at experienced centers.
Glossary
Alloimmunization: A condition in which a pregnant person’s immune system makes antibodies against proteins on the surface of fetal blood cells. This can happen when the mother’s and the fetus’s blood cells are not compatible.
Anemia: A condition caused by a decreased number of red blood cells.
Antibodies: Proteins made by the immune system in response to a foreign substance, such as a virus.
Cell-free DNA: Small fragments of DNA (genetic material) from the placenta that circulate in a pregnant person's blood.
Cordocentesis: A procedure in which a sample of fetal blood is taken from the umbilical cord.
Cytomegalovirus (CMV): A virus that can be passed from a pregnant person to the developing fetus. Babies born with CMV can have problems such as jaundice, anemia, an enlarged liver and spleen, and a skin rash. Babies without symptoms at birth can develop deafness, cognitive difficulties, eye problems, and seizures.
Hemolytic disease of the fetus and newborn (HDFN): A condition where a fetus’s or newborn’s blood cells break down at a faster rate than normal, causing anemia and other complications.
Intrauterine transfusion: A procedure in which donor blood is given to the fetus before birth.
Oxygen: The gas the human body uses to convert food into energy.
Parvovirus: A virus that usually causes mild or no symptoms. Parvovirus infection during pregnancy can be passed to the developing fetus and may cause severe fetal anemia.
Placenta: A special organ that develops during pregnancy. It allows the transfer of nutrients, antibodies, and oxygen to the fetus. It also makes hormones that sustain the pregnancy.
Red blood cells (RBCs): The cells that carry oxygen in the body.
Stillbirth: Death of a fetus prior to delivery.
Ultrasound: Use of sound waves to create images of internal organs or the fetus during pregnancy.
Umbilical cord: The structure linking the growing fetus to the placenta; it contains blood vessels that bring oxygen and nutrients to the fetus and remove waste products.
Last reviewed: September 2026