What is the most common cause of raised unconjugated bilirubin in newborns within the first 24 hours of life?

The most common cause of raised unconjugated bilirubin in newborns within the first 24 hours of life is indeed related to ABO incompatibility. In cases of ABO incompatibility, the mother’s immune system can produce antibodies against the baby’s blood type if they are different. This typically happens when an O-type mother has a baby with a blood type A or B. These specific antibodies can lead to the rapid destruction of the baby's red blood cells, resulting in an increased production of unconjugated bilirubin due to hemolysis.

This elevated unconjugated bilirubin can manifest as jaundice, appearing as early as the first 24 hours of life, which is a critical time frame for identifying conditions that can lead to hyperbilirubinemia in newborns. The quick onset differentiates it from other causes of jaundice, such as physiologic jaundice, which commonly develops after the first day of life and resolves within a few days, or breast milk jaundice, which typically occurs after the first week of life. Hepatitis infection is also less common in the immediate newborn period and presents with additional symptoms.

This understanding is crucial for medical professionals to ensure timely diagnosis and management of hyperbilirubinemia in newborns.