Neonatal jaundice
Newborns generate more bilirubin and clear it less efficiently than adults, so mild jaundice after the first day is usually physiological, but the timing and conjugation status of the bilirubin are what separate this from pathology capable of crossing into the brain.
First principles
Newborns are set up to make more bilirubin and clear it more slowly
Neonates have a higher red cell mass with red cells that have a shorter lifespan than adult cells, so haem breakdown generates more bilirubin per kilogram than at any later point in life. At the same time, hepatic glucuronyl transferase (the enzyme that conjugates bilirubin so it can be excreted in bile) is physiologically immature in the first days of life, so unconjugated bilirubin is produced faster than it can be cleared. This combination of increased production and reduced clearance is why mild jaundice appearing after 24 hours of age and resolving within two weeks is normal and expected, not a disease process.
Educational content pending clinical review. Not medical advice.