Abstract
<title>Abstract</title> <p> <bold>Background</bold> This study aimed to investigate the dynamic changes in foetal haemoglobin (HbF) in the early postnatal period and their correlation with end-tidal carbon monoxide corrected to ambient carbon monoxide (ETCOc) values. <bold>Methods</bold> A prospective study enrolled 103 late preterm and term small-for-gestational-age (SGA) infants, who were assigned to haemolytic and non-haemolytic groups. ETCOc and HbF were measured at postnatal day 0 (within 24 h) and day 3 (72–96 h), with subsequent statistical analysis. <bold>Results</bold> Levels of both ETCOc and HbF decreased over time with postnatal age in neonates. A significant positive correlation was detected between ETCOc and HbF in non-haemolytic infants, whereas no significant correlation was observed in infants with haemolytic disease. The differential correlation patterns between the two groups remained consistent at both 24 h and postnatal day 3. <bold>Conclusion</bold> Elevated HbF levels in the early postnatal period may serve as an important risk factor for neonatal hyperbilirubinemia, particularly in infants with non-haemolytic jaundice. </p>