Babies are born exhibiting the full range of human variation. They differ with respect to weight, length, head size, fatness, trunk and limb circumferences, skin color, hair (color and absence/presence), personality, and in innumerable other ways. They also vary with respect to age. Although the way we assign ages to individuals is based on everyone starting at age 0 at birth, in reality newborns are already of different ages, some having been in utero for over forty-two weeks, some as little as half of that!

In the United States, certain characteristics of newborns are particularly closely attended to by family members and friends. The first questions asked about newborns are “Boy or girl?” “How much did he/she weigh?” and, less often, “How long was he/she?” These statistics appear on birth announcements, which is the only time in the lifecycle when the specifics of one's size are touted so publicly. To medical personnel, other measures are equally important. The APGAR score, done at one and five minutes after birth, assesses color, pulse, reflex irritability, muscle tone, and respiratory effort. Blood samples may be taken to ascertain glucose and bilirubin status. All of these measures are culturally and medically significant in their own ways and generally help ascertain the health status of the newborn.

The newborn measurement most often used as a summary index of a newborn's condition is weight, but other measures provide different or more specific information.