In basic English usage, an infant is defined as a human child at the youngest stage of life, especially before they can walk or simply a child before the age of one[1] (see also child and adolescent).

The term "infant" derives from the Latin word in-fans, meaning "unable to speak." There is no exact definition for infancy. "Infant" is also a legal term with the meaning of minor[2]; that is, any child under the age of legal adulthood.

A human infant less than a month old is a newborn infant or a neonate.[3] The term "newborn" includes premature infants, postmature infants and full term newborns.

Quite often babies are called infants until they reach the age of one. Babies are traditionally called "toddlers" (generally 12-24 months) when they start to walk, whether or not they have reached this age. Daycares with an "infant room" often call all children in it "infants" even if they are older than a year and/or walking; they sometimes use the term "walking infant".

Appearance


A newborn's shoulders and hips are narrow, the abdomen protrudes slightly, and the arms and legs are relatively short. The average birth weight of a full-term newborn is approximately 7 ½ pounds (3.2 kg), but is typically in the range of 5.5–10 pounds (2.7–4.6 kg). The average total body length is 14–20 inches (35.6–50.8 cm), although premature newborns may be much smaller. The Apgar score is a measure of a newborn's transition from the womb during the first minutes of life.

A newborn's head is very large in proportion to the rest of the body, and the cranium is enormous relative to his or her face. While the adult human skull is about 1/8 of the total body length, the newborn's is about 1/4. At birth, many regions of the newborn's skull have not yet been converted to bone, leaving "soft spots" known as fontanels. The two largest are the diamond-shaped anterior fontanel, located at the top front portion of the head, and the smaller triangular-shaped posterior fontanel, which lies at the back of the head. Later in the child's life, these bones will fuse together in a natural process. A protein called noggin is responsible for the delay in an infant's skull fusion.[4]

During labour and birth, the infant's skull changes shape to fit through the birth canal, sometimes causing the child to be born with a misshapen or elongated head. It will usually return to normal on its own within a few days or weeks. Special exercises sometimes advised by physicians may assist the process.

Some newborns have a fine, downy body hair called lanugo. It may be particularly noticeable on the back, shoulders, forehead, ears and face of premature infants. Lanugo disappears within a few weeks. Likewise, not all infants are born with lush heads of hair. Some may be nearly bald while others may have very fine, almost invisible hair. Some babies are even born with a full head of hair. Amongst fair-skinned parents, this fine hair may be blond, even if the parents are not. The scalp may also be temporarily bruised or swollen, especially in hairless newborns, and the area around the eyes may be puffy.

Immediately after birth, a newborn's skin is often grayish to dusky blue in color. As soon as the newborn begins to breathe, usually within a minute or two, the skin's color returns to its normal tone. Newborns are wet, covered in streaks of blood, and coated with a white substance known as vernix caseosa, which is hypothesised to act as an antibacterial barrier. The newborn may also have Mongolian spots, various other birthmarks, or peeling skin, particularly on the wrists, hands, ankles, and feet.

A newborn's genitals are enlarged and reddened, with male infants having an unusually large scrotum. The breasts may also be enlarged, even in male infants. This is caused by naturally-occurring maternal hormones and is a temporary condition. Females (and even males) may actually discharge milk from their nipples (sometimes called witch's milk), and/or a bloody or milky-like substance from the vagina. In either case, this is considered normal and will disappear in time.

The umbilical cord of a newborn is bluish-white in color. After birth, the umbilical cord is normally cut, leaving a 1–2 inch stub. The umbilical stub will dry out, shrivel, darken, and spontaneously fall off within about 3 weeks. Occasionally, hospitals may apply triple dye to the umbilical stub to prevent infection, which may temporarily color the stub and surrounding skin purple.

Newborns lose many of the above physical characteristics quickly. Thus prototypical older babies look very different. While older babies are considered "cute", newborns can be "unattractive" by the same criteria and first time parents may need to be educated in this regard.