A 3-day-old term Indian neonate born to an O Rh-positive mother presents with clinical jaundice. Serum bilirubin is 12 mg/dL. What is the most common cause of neonatal jaundice in the first week of life?

Explanation

## Epidemiology of Neonatal Jaundice **Key Point:** Physiological jaundice is the most common cause of neonatal jaundice, occurring in 50–60% of term and 80% of preterm neonates. ### Mechanism of Physiological Jaundice Physiological jaundice results from: 1. Increased bilirubin production (shorter RBC lifespan: 70–90 days vs. 120 days in adults; immature reticuloendothelial system) 2. Decreased hepatic uptake and conjugation of bilirubin (low UGT1A1 enzyme activity at birth) 3. Increased enterohepatic circulation (high intestinal β-glucuronidase activity) ### Timing and Clinical Features | Feature | Physiological Jaundice | |---------|------------------------| | **Onset** | After 24 hours of life | | **Peak** | Day 3–5 in term infants | | **Duration** | Resolves by 1–2 weeks | | **Bilirubin rise** | <0.2 mg/dL/hour | | **Infant appearance** | Well, feeding normally | **High-Yield:** Physiological jaundice is benign and self-limited; it does NOT require treatment unless bilirubin exceeds phototherapy thresholds (which depend on age in hours and risk category). ### Comparison with Other Causes | Cause | Onset | Frequency | Key Feature | |-------|-------|-----------|-------------| | **Physiological** | >24 hrs | 50–60% term | Benign, self-limited | | **Breastfeeding jaundice** | 3–5 days | 10–15% | Poor latch, inadequate intake | | **ABO incompatibility** | <24 hrs | 0.3–1% | Mother O, infant A/B; mild hemolysis | | **Sepsis** | Variable | <1% | Ill-appearing, other signs | **Clinical Pearl:** A well-appearing neonate with jaundice after 24 hours and a bilirubin rise of <0.2 mg/dL/hour is almost certainly physiological jaundice.