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An infant is the more formal or specialised synonym for "baby", the very young offspring of a human. The term may also be used to refer to juveniles of other organisms.
A newborn is, in colloquial use, an infant who is only hours, days, or up to one month old. In medical contexts, newborn or neonate refers to an infant in the first 28 days after birth; the term applies to premature, full term, and postmature infants; before birth, the term "fetus" is used. The term "infant" is typically applied to young children under one year of age; however, definitions may vary and may include children up to two years of age. When a human child learns to walk, the term "toddler" may be used instead.
In British English, an infant school is for children aged between four and seven. As a legal term, "infancy" continues from birth until age 18.
A newborn's shoulders and hips are wide, the abdomen protrudes slightly, and the arms and legs are relatively long with respect to the rest of their body. The Apgar score is a measure of a newborn's transition from the uterus during the first minutes after birth.
In first world nations, the average total body length of newborns are 35.6–50.8 cm , although premature newborns may be much smaller.
Ways to measure a baby's length is to lay him down and stretch a measuring tape from the top of his head to the bottom of his heel. Measure the circumference of your baby's head also.
In developed countries, the average birth weight of a full-term newborn is approximately 3.4 kg , and is typically in the range of 2.7–4.6 kg.
Over the first 5–7 days following birth, the body weight of a term neonate decreases by 3–7%, and is largely a result of the resorption and urination of the fluid that initially fills the lungs, in addition to a delay of often a few days before breastfeeding becomes effective. After the first week, healthy term neonates should gain 10–20 grams/day.
A newborn's head is very large in proportion to the body, and the cranium is enormous relative to his or her face. While the adult human skull is about one seventh of the total body length, the newborn's is about ¼. Normal head circumference for a full-term infant is 33–36 cm at birth. 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.