Why might a murmur not be noted during a newborn's initial exam?

A murmur might not be noted during a newborn's initial exam primarily due to elevated pulmonary vascular resistance, which is typical in newborns. Right after birth, the newborn's circulatory system undergoes significant changes as it transitions from fetal circulation to independent circulation. In the womb, the lungs are not used for oxygen exchange, leading to higher blood pressure in the pulmonary arteries (increased pulmonary vascular resistance).

This elevated resistance can prevent certain blood flow patterns that may cause turbulent flow (which would produce a murmur) from being established. As the baby begins to breathe and oxygen levels increase post-delivery, pulmonary vascular resistance decreases, which may lead to the emergence of murmurs that weren't present immediately after birth.

Understanding this physiological phenomenon illustrates why it’s common not to hear murmurs during the initial examination. The other choices do not accurately capture the physiological processes at play during a newborn's examination and may reflect misunderstandings of normal newborn assessments.