What is the most likely cause of unconjugated hyperbilirubinemia shortly after birth?

Unconjugated hyperbilirubinemia shortly after birth is commonly associated with various causes, but the most likely cause in a newborn is indeed ABO incompatibility. This condition occurs when there is a blood type mismatch between the mother and the baby, typically when the mother has type O blood and the baby has type A or B blood.

In cases of ABO incompatibility, the mother’s immune system creates antibodies against the baby’s red blood cells, leading to hemolysis (the destruction of red blood cells). This process releases hemoglobin, which is then converted into bilirubin, predominantly unconjugated bilirubin, leading to elevated levels in the blood. This can result in jaundice that manifests in the first 24 hours of life, a hallmark feature of ABO incompatibility.

Physiological jaundice, while common in newborns, tends to occur slightly later, usually after the first 24 hours, and is generally considered a normal process of neonatal liver maturation. Breast milk jaundice and hemolysis due to G6PD deficiency also present later in the neonatal period, making them less likely causes of early unconjugated hyperbilirubinemia. Thus, ABO incompatibility clearly aligns with the timing and biochemical characteristics of the jau