A term neonate on day 4 of life has serum total bilirubin of 18 mg/dL with predominantly unconjugated fraction. Mother is blood group O Rh-positive, baby is A Rh-positive. Direct Coombs test is positive. Which of the following features most distinguishes ABO incompatibility from Rh isoimmunization?

• A Rh isoimmunization shows predominantly spherocytes on peripheral smear
• B ABO incompatibility rarely causes hydrops fetalis
• C Rh incompatibility always has a positive direct Coombs test
• D ABO incompatibility can occur in the first pregnancy ✓

Explanation

ABO incompatibility can occur in the first pregnancy because group O mothers have naturally occurring anti-A and anti-B IgG antibodies that can cross the placenta even without prior sensitization. Rh isoimmunization almost never causes severe disease in the first pregnancy as sensitization requires prior exposure. Spherocytes are characteristic of ABO incompatibility (not Rh disease) because anti-A/anti-B antibodies cause extravascular hemolysis producing spherocytes. Hydrops fetalis is actually more characteristic of severe Rh incompatibility than ABO incompatibility.

Reference: Ghai Essential Pediatrics, 10th ed.

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