What is the upper limit for the classification of a newborn?

The definition of a newborn, typically classified as an infant in a medical context, is generally understood to encompass the first 28 days of life. This period is also referred to as the neonatal phase. The upper limit for this classification is set at 1 month old, aligning with standard medical terminology, which emphasizes the biological and developmental milestones that occur during this critical early stage of life.

Choosing 1 month old accurately reflects this convention, distinguishing newborns from older infants who are no longer in the neonatal phase. As such, answering with this option encapsulates the appropriate clinical understanding of a newborn's age classification.