Physiological Jaundice of the Newborn

Definition

Physiological (neonatal) jaundice is the transient, mild unconjugated hyperbilirubinemia that develops in nearly every newborn because the hepatic machinery for bilirubin uptake, conjugation, and excretion does not fully mature until about 2 weeks of age - Robbins & Kumar Basic Pathology, p. 619.

Nearly every newborn develops an unconjugated bilirubin level above 1 mg/dL (the adult upper limit) during the first week of life, and jaundice becomes visible clinically once total bilirubin rises above roughly 5 mg/dL - ROSEN's Emergency Medicine, Foundations.

Pathophysiology

Three physiologic factors combine to produce it - ROSEN's Emergency Medicine:

Increased bilirubin production - neonates have a higher red cell mass with shorter-lived, more fragile red cells, so hemoglobin turnover and heme breakdown are greater than in adults.
Decreased hepatic clearance and conjugation - the enzyme bilirubin-UDP-glucuronosyltransferase (which conjugates bilirubin so it can be excreted in bile) is present at low activity at birth and takes about two weeks to mature - Harper's Illustrated Biochemistry.
Increased enterohepatic resorption - the neonatal gut reabsorbs more unconjugated bilirubin back into the circulation instead of eliminating it in stool.

Before birth, fetal bilirubin is cleared across the placenta by the maternal liver. Immediately after delivery the neonatal liver must suddenly take over this job on its own, and it is not yet fully equipped to do so - Harrison's Principles of Internal Medicine, 22nd ed.

Breastfeeding can exaggerate the picture: enzymes in breast milk (glucuronidases) deconjugate bilirubin in the gut, increasing enterohepatic recycling and prolonging jaundice ("breast milk jaundice") - Robbins & Kumar Basic Pathology; ROSEN's Emergency Medicine.

Key clinical features

Onset typically after 24 hours of life (appearance in the first 24 hours points away from a physiologic cause and toward a pathologic one, e.g., hemolytic disease).
Peaks around day 3-5 in term infants (later, and to higher levels, in preterm infants because hepatic immaturity is more pronounced).
Usually resolves by 1-2 weeks in term infants.
Bilirubin is predominantly unconjugated (indirect); conjugated hyperbilirubinemia in a neonate is uncommon and is always considered pathologic, warranting investigation - ROSEN's Emergency Medicine.
Because unconjugated bilirubin is fat-soluble and tightly albumin-bound, it does not appear in urine, but at high unbound levels it can cross into the brain (particularly in preterm or unwell infants) and cause kernicterus, a severe, potentially irreversible neurologic injury - Robbins & Kumar Basic Pathology.

Management

Phototherapy with blue light converts bilirubin into a water-soluble photoisomer that can be excreted without conjugation, and it is sufficient in most cases to keep bilirubin in a safe range until the infant's own conjugating machinery matures - Robbins & Kumar Basic Pathology. Exchange transfusion is reserved for severe or rapidly rising hyperbilirubinemia at risk of kernicterus.

Distinguishing physiologic from pathologic jaundice

Pathologic causes to exclude include hemolytic disease (Rh/ABO incompatibility, G6PD deficiency, spherocytosis), sepsis, polycythemia, birth trauma with sequestered blood, Crigler-Najjar and Gilbert syndromes, and biliary/hepatic disease. Red flags include jaundice within the first 24 hours of life, conjugated (direct) hyperbilirubinemia, rapidly rising bilirubin, or jaundice persisting beyond 2 weeks - ROSEN's Emergency Medicine; Quick Compendium of Clinical Pathology.

A recent evidence check for this topic (meta-analyses/systematic reviews, last 2 years) was not run in this session since the physiology here is well-established and unlikely to have changed; if you want, I can pull current AAP/NICE bilirubin treatment thresholds or recent phototherapy trial data.

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