Can an ABO (ABO blood group) positive mother have an Rh (Rhesus) negative baby?

Can an AB Positive Mother Have an Rh Negative Baby?

Yes, an AB positive mother can have an Rh negative baby if the father carries an Rh negative gene. Blood group inheritance follows Mendelian genetics, with the Rh factor and ABO blood groups being inherited independently.

Blood Group Inheritance Patterns

ABO Blood Group Inheritance

• The ABO blood group is determined by three alleles: A, B, and O
• A and B alleles are codominant (both expressed when present together)
• O allele is recessive to both A and B
• An AB positive mother has both A and B alleles

Rh Factor Inheritance

• The Rh factor is determined by a single gene with two alleles: Rh positive (D) and Rh negative (d)
• Rh positive (D) is dominant over Rh negative (d)
• An Rh positive person can be either homozygous (DD) or heterozygous (Dd)
• An Rh negative person must be homozygous recessive (dd)

How an AB Positive Mother Can Have an Rh Negative Baby

For an AB positive mother to have an Rh negative baby:

1. The mother must be heterozygous for the Rh factor (Dd)
2. The father must contribute an Rh negative allele (d)

If the mother is AB positive with genotype Dd (heterozygous for Rh factor) and the father contributes a d allele, there is a 50% chance the baby will be Rh negative.

Clinical Implications

This genetic possibility has important clinical implications:

1. Risk of Rh Alloimmunization in Future Pregnancies:

   • If an Rh negative baby is born to an Rh positive mother, there is no risk of Rh incompatibility in that pregnancy
   • However, if a subsequent pregnancy involves an Rh positive fetus, and the mother had previously been exposed to Rh positive blood, there could be a risk of alloimmunization

2. Prevention of Rh Alloimmunization:

   • Anti-D immunoglobulin (RhIg) is given to Rh negative mothers carrying Rh positive fetuses to prevent alloimmunization 1
   • This is not necessary when an Rh positive mother delivers an Rh negative baby

Frequency of Blood Type Combinations

According to research on blood type incompatibilities:

• The total number of births to ABO Rh D negative mothers is approximately 14% 2
• The emergence of immune antibodies in Rh D negative mothers is about 1% 2
• ABO incompatibilities occur in approximately 2.3% of pregnancies 2

Conclusion

The inheritance of blood type and Rh factor follows specific genetic patterns that allow an AB positive mother to have an Rh negative baby. Understanding these inheritance patterns is crucial for appropriate management of pregnancies and prevention of hemolytic disease of the newborn in cases where there is potential for Rh incompatibility.