The term newborn is often used for babies under 28 days or even up to 2 or 3 months of age, according to experts. After this period (and even during the newborn stage), your little one may be referred to as a baby or an infant up until they're around 1 year of age.

In medical and clinical terms, as defined by organizations like the World Health Organization (WHO) and the American Academy of Pediatrics (AAP), the newborn stage (or neonatal period) strictly refers to the first 28 days of a child's life. During these critical four weeks, the infant is undergoing the most rapid physiological changes as they adapt to life outside the womb. However, in developmental psychology and general parenting contexts, the "newborn stage" is often colloquially extended to cover the first 2 to 3 months of life, ending when the baby begins to reach the "infant" milestones such as intentional smiling, improved neck control, and more predictable sleep patterns. Many experts also refer to this extended window as the "Fourth Trimester," emphasizing that the baby still requires a womb-like environment of constant closeness, warmth, and frequent feeding. By the time a baby reaches 12 weeks, they typically transition out of the newborn phase and into a more active infancy marked by greater engagement with their surroundings.