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Blood | Fetal Anaemia

Fetal anaemia 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 anaemia 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 anaemia is diagnosed, a fetal blood transfusion should be considered.


Anaemia 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 anaemic. One of the most common disorders to causes fetal anaemia is Rhesus alloimmunization. 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, where they can destroy the red blood cells causing the fetus to become anaemic.

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

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

When a fetus is severely anaemic, 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 (ascites or hydrops). Fetuses in this condition are very sick and, without treatment, may not survive.

Fetal anaemia 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 anaemic is the fetus.

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

When fetal anaemia is suspected, blood taken from the mother may provide some important information about the presence of antibodies that can lead to fetal anaemia, 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 fine 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 anaemia can be made.



When severe fetal anaemia is diagnosed, intrauterine fetal transfusion may be considered. In experienced hands, intrauterine transfusions are successful in more than 97% 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 performed at the same time as a fetal blood sampling.


Referral Information


  • Fetal anaemia 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 (MoM).

Interventions are typically performed as outpatient procedures. Repeated procedures may be necessary (every 2-3 weeks). Post-operatively, out-of-province or distant patients will be referred back to their local care provider for ongoing 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.