Blood | Fetal Anemia

Fetal anemia refers to an insufficient number of red blood cells in the fetal blood circulation. Just like in adults, red blood cells in the fetus are responsible for transporting oxygen to the tissues. If there are insufficient red blood cells, the fetus may develop heart failure.

Essential Information

  • Fetal anemia is caused by a lack of red blood cells in the fetal circulation.
  • Red blood cells are responsible for transporting oxygen to the fetal tissues. When there are insufficient numbers of red blood cells in the fetal circulation, the fetal heart may overcompensate and beat harder and faster and the fetus may develop heart failure.
  • When severe fetal anemia is diagnosed, a fetal blood transfusion may be considered.


Anemia is a lack of red blood cells in the fetal circulation. The red blood cells carry oxygen from the placenta to the fetal tissues.

Different conditions can cause the fetus to become anemic. One of the most common disorders to causes fetal anemia is Rhesus Iso-immunization. In this disease, the mother and fetal blood types are incompatible. The mother’s immune system therefore recognizes the fetal red blood cells as foreign and mounts an immune attack by forming antibodies to the fetal cells. These antibodies can cross through the placenta and reach the fetal circulation. In this way, they can destroy the red blood cells and cause the fetus to become anemic.

Other less common reasons for fetal anemia are fetal infections with parvovirus B19, or bleeding from the fetal circulation into the mother’s circulation (called “fetomaternal hemorrhage”). Some inborn blood disorders can also lead to fetal anemia, as can fetal tumours called “sacrococcygeal teratomas”.

Mild and moderate anemia may not affect the fetal wellbeing and the long-term outcome of these infants can be excellent.

When a fetus is severely anemic, however, there will be insufficient red blood cells to sustain adequate oxygen transport to the tissues. The fetus will try to compensate for this by increasing its heart function, and circulating the available red blood cells faster. Eventually, however, the heart will not be able to maintain this high output and heart failure will develop. This may be seen by studying the heart’s appearance on a focused ultrasound (called a fetal echocardiogram) but may also become evident by the accumulation of fluid under the fetal skin or in the fetal abdomen or chest (hydrops). Fetuses with this condition are very sick and may not survive.

Fetal anemia can be indirectly diagnosed by a targeted ultrasound that measures the speed of the blood flow through one of the small vessels in the fetal brain, called the middle cerebral artery (MCA). The faster the blood flow in the MCA, the more anemic the fetus.

Impending heart failure may be diagnosed on ultrasound or fetal echocardiography, and hydrops can be diagnosed on ultrasound.

When fetal anemia is suspected, blood taken from the mother may provide some important information about the presence of antibodies that can lead to fetal anemia, or may suggest that the mother was recently exposed to Parvovirus B19, for example.

Fetal blood sampling refers to actually obtaining a sample of the fetal blood and counting the number of red blood cells, similar to a blood test in an adult. Under ultrasound guidance, a needle is inserted through the maternal abdominal wall into the womb, and a sample is obtained from one of the fetal blood vessels. This sample can then be analyzed in the lab and a definite diagnosis of fetal anemia can be made.



When severe fetal anemia is diagnosed, intrauterine fetal transfusion may be considered. In experienced hands, intrauterine transfusions are successful in more than 97 per cent of cases. Guided by ultrasound, the fetus receives a blood transfusion via the umbilical vein. This may need to be repeated multiple times throughout the course of the pregnancy. The intrauterine fetal transfusion is often performed at the same time as a fetal blood sampling. If anemia is diagnosed in the sample obtained from the fetus, a transfusion is performed.


Referral Information (for physicians)

Inclusion criteria:

  • Fetal anemia due to red blood cell alloimmunization (confirmed red blood cell antibodies; incompatible red blood cell phenotype in partner) or Parvovirus B19 infection (Parvovirus IgM positive)
  • Middle cerebral artery peak systolic velocity >1.5 multiples of the median.


interventions are typically performed as outpatient procedures. Repeated procedures may be necessary (every 2-3 weeks). Postoperatively, out-of-province patients will be referred back to their local care provider for further antenatal care and delivery. Local patients will have shared care between the referring centre and Mount Sinai Hospital. Delivery will be planned at Mount Sinai Hospital.