What is the primary cause of physiologic jaundice in newborns?

The primary cause of physiologic jaundice in newborns is indeed the destruction of fetal red blood cells. In the neonatal period, particularly during the first few days after birth, infants experience a breakdown of the fetal red blood cells that were necessary during gestation. This process releases bilirubin, a byproduct of hemoglobin breakdown, into the bloodstream.

Newborns have a limited capacity to conjugate bilirubin due to the immaturity of their liver function. So while liver immaturity plays a role in the duration and severity of jaundice, the initial trigger for physiologic jaundice is primarily the increased destruction of red blood cells, leading to elevated levels of unconjugated bilirubin.

This process is considered physiologic because it is a normal adaptation as the infant transitions from intrauterine to extrauterine life, and typically resolves within the first couple of weeks as the liver matures and the normal balance of bilirubin production and excretion is established.