When are babies no longer newborns? The Centers for Disease Control and Prevention (CDC) classifies babies as newborns during the first 28 days of life. After this period, they are considered infants. The American Academy of Pediatrics (AAP) and other medical organizations consider babies to be newborns until they reach 4 weeks of age. Non-profit organizations like First Candle provide support and resources for newborns and their families.