Erythroblastosis fetalis is hemolytic anemia in the fetus or neonate caused by transplacental transmission of maternal antibodies to fetal RBCs. The disorder usually results from incompatibility between maternal and fetal blood groups, often Rh₀(D) antigens. Diagnosis begins with prenatal maternal antigenic and antibody screening and may require paternal screening, serial measurement of maternal antibody titers, and fetal testing. Treatment may involve intrauterine fetal transfusion or neonatal exchange transfusion. Prevention is Rh₀(D) immune globulin injection for women at risk.

Erythroblastosis fetalis classically results from Rh₀(D) incompatibility, which may develop when a woman with Rh-negative blood is impregnated by a man with Rh-positive blood and conceives a fetus with Rh-positive blood (see also Perinatal Hematologic Disorders: Hemolysis). Other fetomaternal incompatibilities that can cause erythroblastosis fetalis involve the Kell, Duffy, Kidd, MNSs, Lutheran, Diego, Xg, P, Ee, and Cc antigen systems, as well as other antigens. Incompatibilities of ABO blood types do not cause erythroblastosis fetalis.

Pathophysiology

Fetal RBCs normally move across the placenta to the maternal circulation throughout pregnancy. Movement is greatest at delivery or termination of pregnancy. Movement of large volumes (eg, 10 to 150 mL) is considered significant fetomaternal hemorrhage; it can occur after trauma and sometimes after delivery or termination of pregnancy. In women who have Rh-negative blood and who are carrying a fetus with Rh-positive blood, fetal RBCs stimulate maternal antibody production against the Rh antigens. The larger the fetomaternal hemorrhage, the more antibodies produced. The mechanism is the same when other antigen systems are involved; however, Kell antibody incompatibility also directly suppresses RBC production in bone marrow.

Other causes of maternal anti-Rh antibody production include injection with needles contaminated with Rh-positive blood and inadvertent transfusion of Rh-positive blood.

No complications develop during the initial sensitizing pregnancy; however, in subsequent pregnancies, maternal antibodies cross the placenta and lyse fetal RBCs, causing anemia, hypoalbuminemia, and possibly high-output heart failure or fetal death. Anemia stimulates fetal bone marrow to produce and release immature RBCs (erythroblasts) into fetal peripheral circulation (erythroblastosis fetalis). Hemolysis results in elevated indirect bilirubin levels in neonates, causing kernicterus (see Metabolic, Electrolyte, and Toxic Disorders in Neonates: Kernicterus). Usually, isoimmunization does not cause symptoms in pregnant women.

Diagnosis

• Maternal blood and Rh typing and reflex antibody screening
• Serial antibody level measurements and sometimes middle cerebral artery blood flow measurements for pregnancies at risk

At the first prenatal visit, all women are screened for blood type, Rh type, and anti-Rh₀(D) and other antibodies that are formed in response to antigens and that can cause erythroblastosis fetalis (reflex antibody screening). If women have Rh-negative blood and test positive for anti-Rh₀(D) or they test positive for another antibody that can cause erythroblastosis fetalis, the father's blood type and zygosity (if paternity is certain) are determined. If he has Rh-negative blood and is negative for the antigen corresponding to the antibody identified in the mother, no further testing is necessary. If he has Rh-positive blood or has the antigen, maternal anti-Rh antibody titers are measured. If titers are positive but less than a laboratory-specific critical value (usually 1:8 to 1:32), they are measured monthly until 24 wk, then every 2 wk. If the critical value is exceeded, fetal middle cerebral artery blood flow is measured at intervals of 1 to 2 wk depending on titers and patient history; the purpose is to detect high-output heart failure, indicating high risk of anemia. Elevated blood flow for gestational age should prompt percutaneous umbilical blood sampling to obtain a sample of fetal blood. If paternity is reasonably certain and the father is likely to be heterozygous for Rh₀(D), the fetus's Rh type is determined. If fetal blood is Rh positive or status is unknown and if middle cerebral artery flow is elevated, fetal anemia is likely.

Treatment

• Fetal blood transfusions
• Delivery at 32 to 34 wk

If fetal blood is Rh negative or if middle cerebral artery blood flow remains normal, pregnancy can continue to term untreated. If fetal anemia is likely, the fetus can be given intravascular intrauterine blood transfusions by a specialist at an institution equipped to care for high-risk pregnancies. Transfusions occur every 1 to 2 wk until fetal lung maturity is confirmed (usually at 32 to 34 wk), when delivery should be done. Corticosteroids should be given before the first transfusion if the pregnancy is > 24 wk, possibly > 23 wk.

Neonates with erythroblastosis are immediately evaluated by a pediatrician to determine need for exchange transfusion (see Perinatal Hematologic Disorders: Exchange transfusion).

Prevention

Prevention involves giving the mother

• Rh₀(D) immune globulin at 28 wk gestation and within 72 h of pregnancy termination

Delivery should be as atraumatic as possible. Manual removal of the placenta should be avoided because it may force fetal cells into maternal circulation.

Maternal sensitization and antibody production due to Rh incompatibility can be prevented by giving the woman Rh₀(D) immune globulin. This preparation contains high titers of anti-Rh antibodies, which neutralize Rh-positive fetal RBCs. Because fetomaternal transfer and likelihood of sensitization is greatest at termination of pregnancy, the preparation is given within 72 h after termination of each pregnancy, whether by delivery, abortion, or treatment of ectopic pregnancy. The standard dose is 300 μg IM. A rosette test can be used to rule out significant fetomaternal hemorrhage, and if results are positive, a Kleihauer-Betke (acid elution) test can measure the amount of fetal blood in the maternal circulation. If test results indicate fetomaternal hemorrhage is massive (> 30 mL whole blood), additional injections (300 μg for every 30 mL of fetal whole blood, up to 5 doses within 24 h) are necessary.

Treatment at termination of pregnancy is occasionally ineffective because sensitization occurred earlier during pregnancy. Therefore, at about 28 wk, all pregnant women with Rh-negative blood and no known prior sensitization are given a dose. Some experts recommend a 2nd dose if delivery has not occurred by 40 wk. Rh₀(D) immune globulin should also be given after any episode of vaginal bleeding and after amniocentesis or chorionic villus sampling. Anti-Rh antibodies persist for > 3 mo after one dose.

Last full review/revision February 2010 by Antonette T. Dulay, MD

Content last modified February 2010