What is the first step in measuring an infant's length correctly?

To measure an infant's length accurately, the correct procedure begins with placing the infant supine on a flat surface. This method ensures the infant is positioned safely and allows for a precise measurement. When the infant lies flat on their back, it is easier to ensure that their body is fully extended, which is crucial for obtaining an accurate length measurement from the top of the head to the heel of the foot.

Positioning the infant this way eliminates variables that could impact the measurement, such as movement or improper posture, which can occur in other positions like sitting upright or being held. Additionally, doing so allows the healthcare professional to use a standard measuring board or straight edge, ensuring that the measurement is both consistent and reliable.