Which factor contributes to physiologic anemia of the newborn shortly after birth?

Physiologic anemia of the newborn is a common occurrence where a newborn shows a drop in hemoglobin levels during the first few weeks of life. This phenomenon largely results from the transition from intrauterine to extrauterine life, where the demand for and production of red blood cells changes.

The correct answer, decreased erythropoietin due to cessation of uterine hypoxia, explains that when the neonate is born, it transitions from a hypoxic environment in utero (where low oxygen levels stimulate the production of erythropoietin, which in turn encourages the production of red blood cells) to an oxygen-rich environment. As the newborn begins breathing air, the need and stimulus for increased red blood cell production diminishes, leading to lower levels of erythropoietin and consequently, a decrease in red blood cell production.

Thus, without the stimulus of hypoxia, there is a natural reduction in erythropoietin levels, contributing to the physiologic anemia seen in newborns shortly after birth. This process is a normal part of adaptation to life outside the womb and is temporary, as the body gradually adjusts to its new environment and red blood cell production stabilizes.