ABO Isoimmunization in Pregnancy: ICD-10-CM Coding Quick Reference

ABO isoimmunization occurs when maternal IgG anti-A or anti-B antibodies cross the placenta and hemolyze fetal red cells expressing the corresponding antigen. It is generally milder than Rh(D) disease but can cause significant neonatal jaundice and anemia.

ICD-10-CM Category O02 — ABO Isoimmunization

Code	Description	Clinical Context
O02.0	ABO isoimmunization, anti-A	Mother group O or B; fetus group A
O02.1	ABO isoimmunization, anti-B	Mother group O or A; fetus group B
O02.2	ABO isoimmunization, anti-A,B	Mother group O; fetus group AB
O02.3	ABO isoimmunization, unspecified	Antibody specificity not documented

Coding Guidelines

• Assign code from category O02 for the maternal record when isoimmunization complicates pregnancy.
• For the newborn, use P55.0 (Rh isoimmunization) or P55.1 (ABO isoimmunization) under "Hemolytic disease of fetus and newborn."
• Do not use O02 codes for Rh isoimmunization (use O36.0-).
• Document antibody titer and fetal monitoring (MCA Doppler, amniotic fluid ΔOD450) to support medical necessity.

Clinical Pearls

• ABO HDFN typically presents in the first pregnancy (unlike Rh, which requires sensitization).
• Direct antiglobulin test (DAT) on cord blood is often weakly positive or negative due to low antigen density on fetal RBCs.
• Management: Phototherapy, IVIG, exchange transfusion rarely needed.

Accurate coding captures severity, guides surveillance, and ensures appropriate reimbursement for high-risk obstetric care.