A baby is medically considered a newborn (or neonate) for the first 28 days (4 weeks) of life, according to the World Health Organization (WHO) and American Academy of Pediatrics (AAP)

. However, in common usage and among many experts, the newborn stage can extend up to 2 or even 3 months of age

. After this period, the baby is typically referred to as an infant, which covers the age from birth up to 1 year

. The term "newborn" is somewhat flexible and can be used informally for up to 3 months, especially as parents adjust to the early developmental stages

. In summary:

• Newborn : Birth to 28 days (medically defined), often extended to 2-3 months (informally)
• Infant : Birth to 1 year
• Toddler : 1 to 3 years

This range reflects both clinical definitions and everyday parenting language.