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

. However, many experts and parents informally extend the newborn phase up to 2 or even 3 months of age, as babies continue to exhibit typical newborn characteristics and developmental milestones during this period

. In summary:

• Strict medical definition: Newborn = birth to 28 days old.
• Common informal usage: Newborn = birth to 2 or 3 months old.
• After this, the baby is usually referred to as an infant until about 1 year old

Thus, while the newborn stage officially lasts about one month, many consider babies to be newborns for up to three months based on developmental and behavioral traits.