A 20-day-old female baby, formerly a 34-week premature infant in the NICU, presents with decreased hemoglobin (7.1 g/dL), indirect hyperbilirubinemia (6.2 mg/dL), and reticulocytosis (9.5%) consistent with hemolytic anemia, but the remainder of the complete blood count (CBC) is normal. The maternal blood type is O and baby has blood type A . A direct antiglobulin test (DAT, direct Coombs) is negative. There is no evidence of blood loss on exam or previous imaging. The baby is hemodynamically stable, gaining weight, and weaning off supplemental oxygen therapy.