Baby: Very young offspring of a human

Other terms can be used to describe the baby's stage of development. These terms do not follow clear rules and are used differently by different people, and in different regions. For example, infant may be used until the baby can walk, while some use "infant" until the baby is one year old. From birth until 3 months of age, a baby can be called a newborn. Depending on how many weeks gestation at birth, newborns may be called premature, post-mature, or full term. The word infant comes from Latin: infans means "unable to speak".

At birth, many parts of the newborn's skull are not yet converted to bone, leaving "soft spots". Later in the child's life, these bones join together naturally. A protein called noggin is responsible for the delay in an infant's skull fusion. During labour and birth, the infant's skull changes shape as it goes through the birth canal. Sometimes this causes the child to be born with a misshapen or elongated head. It will usually return to normal in a short time.

Some religions have ceremonies that occur after a new baby is born, such as Baptism, which involves partly or fully covering the baby in water. After learning to walk, babies are often called toddlers, usually between one and three years of age. Sometimes, a woman's baby may die before or during its birth. Babies which have died in this way are called stillborn babies, or miscarried babies.