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Management of Pregnancy with ABO Incompatibility

Author: Michael T. Mennuti, MD

Editor: Natalie Bowersox, MD

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ABO incompatibility is the most common maternal-fetal blood group incompatibility and the most common cause of hemolytic disease of the newborn (HDN) with an occurrence of 0.5-1% of newborns. ABO incompatibility is more often seen in newborns who have type A blood, and   . in individuals of African or Latin American ethnicity. ABO incompatibility in the newborn generally presents as neonatal jaundice due to a Coombs positive hemolytic anemia. In contrast to the severe intrauterine or neonatal hemolytic anemia associated with Rh sensitization, clinically important neonatal anemia due to ABO incompatibility occurs infrequently. The major clinical issue with HDN due to ABO incompatibility is jaundice.

Several reasons have been proposed to account for the lack of intrauterine hemolysis due to ABO incompatibility. These include less well-developed A and B antigens on fetal red blood cells to stimulate maternal antibody production and the ubiquitous distribution of A and B antigens in other tissues resulting in fewer antibodies that cross the placenta to bind to antigens on fetal red cells. The most important reason that ABO incompatibility does not cause hydrops fetalis is that the naturally occurring anti-A and anti-B antibodies are IgM and do not cross the placenta.

 Less than 1% of mothers with type O have clinically significant anti-A or anti-B antibody that is IgG. ABO incompatibility with transplacental transfer of IgG anti-A antibody, or more commonly, the anti-B antibody has rarely been reported in association with intrauterine hemolysis leading to hydrops fetalis. Other causes of nonimmune hydrops fetalis have not been systematically excluded in these case reports. The most likely explanation for these rare cases is that exposure to antigens similar to the A or B antigen may occur from sources other than the fetus and stimulates IgG production in the mother. For example, an antigen similar to the B antigen is found in E.coli, and exposure to this organism may stimulate the production of IgG anti-B antibody in type O individuals.

In contrast to Rh incompatibility, which tends to become more severe with each subsequent Rh- positive pregnancy, ABO incompatibility does not demonstrate any consistent pattern. Thus, the patient’s first offspring may have the clinically important hemolytic disease of the newborn due to ABO incompatibility, while subsequent newborns may be unaffected or very mildly affected. Because of the rarity of severe intrauterine hemolysis due to ABO incompatibility, assessment for intrauterine fetal anemia is not recommended based on the mother having type O blood. Measurement of IgG anti-A and anti-B antibody may be considered part of the evaluation of unexplained signs of fetal anemia such as ascites or hydrops when the mother is type O. Collecting a cord blood sample at birth for blood type and direct antibody testing should be considered when the mother is type O and a previous child had hemolytic disease of the newborn due to ABO incompatibility.


Further Reading:

American College of Obstetricians and Gynecologists (ACOG), ACOG Practice Bulletin No. 192: Management of Alloimmunization During Pregnancy.  Obstet Gynecol. 2018 Mar;131(3):e82-e90. doi: 10.1097/AOG.0000000000002528.

Koelewijn JM, Vrijkotte TG, van der Schoot CE, Bonsel GJ, et al. Effect of screening for red cell antibodies, other than anti-D, to detect hemolytic disease of the fetus and newborn: a population study in the Netherlands. Transfusion. 2008 May;48(5):941-52. doi: 10.1111/j.1537-2995.2007.01625.x. Epub 2008 Feb 1. PMID: 18248570.

Moise KJ Jr, Argoti PS. Management and prevention of red cell alloimmunization in pregnancy: a systematic review. Obstet Gynecol. 2012 Nov;120(5):1132-9. doi: 10.1097/aog.0b013e31826d7dc1. PMID: 23090532.


Initial Approval September 2011; Reviewed January 2017. Revised May 2018. Reaffirmed November 2019; Minor Revision May 2023


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