Average Bilirubin Levels for 8-Week-Old Infants
By 8 weeks of age, healthy term infants should have total serum bilirubin levels below 1-2 mg/dL, with any jaundice persisting beyond 2-4 weeks requiring evaluation for pathologic causes, particularly cholestasis. 1
Normal Bilirubin Trajectory
Physiologic jaundice in healthy term newborns typically peaks at 3-5 days of life and resolves by 2 weeks in formula-fed infants and by 3-4 weeks in breastfed infants 2, 3
Any infant with visible jaundice persisting at or beyond 3 weeks of age requires measurement of total and direct (or conjugated) bilirubin to identify cholestasis 1
At 8 weeks (approximately 56 days), jaundice should have completely resolved in healthy infants, and persistent elevation warrants immediate investigation 1
When to Investigate at 8 Weeks
If an 8-week-old infant has any visible jaundice or elevated bilirubin, obtain fractionated bilirubin levels immediately to distinguish conjugated from unconjugated hyperbilirubinemia 1, 4
Critical thresholds for abnormal direct/conjugated bilirubin:
- If total serum bilirubin is ≤5 mg/dL, a direct or conjugated bilirubin >1.0 mg/dL is abnormal 1
- If direct bilirubin is >50% of total bilirubin, specialist consultation is required 4
Additional workup for prolonged jaundice at 8 weeks:
- Verify results of newborn thyroid and galactosemia screening 1
- Perform urinalysis and urine culture 1
- Evaluate for sepsis if indicated by clinical presentation 1
Common Pitfalls to Avoid
Do not assume that any jaundice at 8 weeks is "normal breastfeeding jaundice" - while breastfed infants may have slightly prolonged physiologic jaundice, it should not persist beyond 3-4 weeks 2, 3
Do not rely on visual assessment alone - always obtain objective bilirubin measurements if jaundice is suspected at this age 1
Do not subtract direct bilirubin from total bilirubin when making clinical decisions - use the total serum bilirubin value for risk assessment 4
Expected Values by Feeding Type
For reference, normal peak values in the newborn period (not at 8 weeks) are:
- Formula-fed infants: 95th percentile is 11.4 mg/dL, 97th percentile is 12.4 mg/dL 2
- Breastfed infants: 95th percentile is 14.5 mg/dL, 97th percentile is 14.8 mg/dL 2
These values apply only to the first 2-4 weeks of life; by 8 weeks, bilirubin should have normalized to adult levels (<1-2 mg/dL) regardless of feeding method 1, 2