The phrase "newborn infants" refers to babies who are very young, usually defined as being within the first month or so after birth. These infants have just entered into the world and are still in the process of adapting to their new environment outside of the womb. They require a lot of care from their parents or caregivers, including feedings every few hours, diaper changes, and regular check-ups with healthcare professionals. Newborn infants go through many developmental milestones during this time as they grow and mature, such as learning to smile, roll over, and eventually crawl and walk.