## Temporal Pattern of Physiological Jaundice **Key Point:** The timing of peak bilirubin in physiological jaundice follows a predictable pattern related to the maturation of hepatic conjugation and changes in bilirubin load. ### Timeline of Physiological Jaundice in Term Infants | Postnatal Age | Bilirubin Level | Clinical Features | |---|---|---| | < 24 hours | Low | Jaundice rare; if present, suggests pathological cause | | Day 2–3 | Rising | Jaundice becomes clinically visible | | **Day 3–5** | **Peak** | **Maximum unconjugated hyperbilirubinemia** | | Day 5–7 | Declining | Gradual improvement as UGT1A1 activity increases | | Day 7–14 | Normal | Resolution in most term infants | | > 14 days | Normal | Persistence suggests pathological jaundice | ### Physiological Basis for Peak at Day 3–5 1. **Increased bilirubin load:** Peak hemolysis occurs at 24–72 hours (RBC lifespan ~70–90 days) 2. **Immature conjugation:** UGT1A1 activity is lowest in the first 48–72 hours 3. **Enterohepatic circulation:** Reaches maximum contribution by day 3–4 4. **Gradual enzyme maturation:** UGT1A1 activity begins increasing after day 2–3 **Clinical Pearl:** Jaundice appearing within the first 24 hours is **always pathological** and requires investigation for hemolysis (ABO/Rh incompatibility), infection, or other causes. Conversely, jaundice persisting beyond 2 weeks in a term infant warrants evaluation for cholestasis or other pathology. **High-Yield:** The phrase "**day 3–5 peak**" is a must-know for NEET PG. It distinguishes physiological from pathological jaundice and guides the timing of phototherapy decisions using nomograms. 
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