Pregnancy Information Complications Rh Incompatibility

Rh incompatibility occurs when a pregnant woman’s immune system creates antibodies against fetal Rh blood factors. Whether or not Rh incompatibility develops depends on the ABO blood type of both the fetus and the mother. Without treatment with RhoGAM antibodies, Rh incompatibility can cause a serious form of fetal anemia known as erythroblastosis fetalis.

ABO Blood Types and Rh Blood Factors

Everyone has an ABO blood type classification: A , B-, etc. The plus or minus at the end of an ABO classification indicates whether or not the Rh blood factor is present-“Rh positive” means that the Rh factor is present, while “Rh negative” means that it isn’t. Rh is a protein found on red blood cells.

Most ABO blood types are Rh positive: 85 percent of the world’s population has an ABO blood type that is Rh positive. Fetal ABO type is determined by the ABO of both the mother and the father. If the mother’s ABO is Rh negative, knowing the ABO type of the father will help determine the risk of Rh incompatibility.

Rh Incompatibility and Maternal Antibodies

If the fetal ABO is Rh positive and the mother’s ABO is Rh negative, a small chance exists that Rh incompatibility will develop. The maternal and fetal blood systems are separate, but in some cases fetal blood may pass through the placenta to the mother. The mother’s immune system will produce antibodies against the Rh. After destroying Rh factors in the mother’s blood stream, the Rh antibodies may also cross the placenta, where they attack fetal blood cells.

Erythroblastosis Anemia and Other Risks

Although the mother will exhibit no symptoms of Rh incompatibility, Rh antibodies adversely affect fetal health. Rh antibodies can cause fetal heart problems, breathing difficulties, jaundice and a form of anemia known as erythroblastosis fetalis.

Erythroblastosis fetalis can cause a number of fetal complications. In severe cases, it may even cause fetal death. After delivery, signs that a baby experienced erythroblastosis fetalis may include jaundice, anemia and edema (swelling). The baby’s liver and spleen may be enlarged. If a previous pregnancy caused Rh incompatibility, erythroblastosis fetalis is more likely in a subsequent pregnancy with an Rh positive fetus.

Treatment of erythroblastosis fetalis may begin in the womb. Medication and intrauterine blood transfusions may be required to control the condition before birth. After birth, blood transfusions may be required to resolve it.

RhoGAM Treatment and Rh Incompatibility

Rh incompatibility may be detected when blood tests reveal the presence of Rh antibodies. If Rh incompatibility is a factor, ultrasound and amniotic fluid testing are used to monitor the fetus.

RhoGAM antibodies are used to prevent Rh incompatibility. Short for Rh-immune globulin, RhoGAM antibodies prevent the mother from reacting to Rh blood factors. Mothers who are Rh negative are typically given RhoGAM antibodies as a precaution during the 28th week of pregnancy, even before the status of the fetal blood is known. RhoGAM is given to the Rh negative mother again after delivery if the infant turns out to be Rh positive.


American College of Obstetricians and Gynecologists. (nd). The Rh factor: How it can affect your pregnancy.

Beers, M. H.,