A baby is considered a newborn for the first 28 days of life. This four-week window, called the neonatal period, is a distinct medical stage defined by both the World Health Organization and standard clinical practice. After day 28, your baby transitions into the broader “infant” category, which lasts until age one.
Why 28 Days Is the Cutoff
The 28-day mark isn’t arbitrary. It reflects the period of most rapid physiological adjustment as a baby transitions from life in the womb to life outside it. During these first four weeks, your baby’s lungs fully take over breathing, the digestive system begins processing food for the first time, and the body learns to regulate its own temperature by burning a special type of fat (called brown fat) that only fetuses and newborns have.
The neonatal period also carries the highest risk of health complications. The WHO further divides it into two phases: the early neonatal period (days 0 through 7) and the late neonatal period (days 8 through 28). The first week is the most vulnerable, which is why hospitals front-load screenings and checkups during that time.
What Happens During the Newborn Phase
Your baby changes dramatically over these 28 days, even if it doesn’t always look that way from the outside.
Weight: Most newborns lose some weight in the first few days after birth, then start gaining it back. A healthy full-term newborn typically gains about a quarter of an ounce per day for every pound of body weight. So an 8-pound baby would gain roughly 2 ounces a day. By two weeks, most babies are back to their birth weight or above.
Sleep: Newborns sleep around 16 hours a day, split roughly in half between daytime and nighttime. About half of that sleep is REM sleep, which is far more than adults experience. Sleep comes in short bursts rather than long stretches, cycling through multiple stages before repeating.
Appearance: Fine body hair that some babies are born with typically disappears within the first few weeks. Skin color, head shape, and other features that may look unusual at birth generally settle into a more permanent state by the end of the newborn period.
Reflexes That Define the Newborn Stage
Newborns arrive with a set of automatic reflexes that serve as signs of healthy neurological development. These aren’t learned behaviors. They’re hardwired responses that fade on their own schedule as the brain matures.
- Moro reflex: A startled “fling” of the arms when the baby feels a sudden movement or loud noise. It peaks during the first month and typically disappears by two months.
- Rooting reflex: When you stroke a newborn’s cheek, they turn toward your hand and open their mouth, searching for food. This lasts about four months.
- Palmar grasp: Touch your baby’s palm and their fingers will curl tightly around yours. This reflex sticks around until five or six months.
- Stepping reflex: Hold a newborn upright with their feet touching a flat surface and they’ll make walking motions. This disappears around two months, then reappears near the end of the first year as actual walking develops.
Not every baby acquires and loses these reflexes on exactly the same timeline, but they’re present in some form in virtually all healthy newborns.
Screenings in the First 28 Days
Several standard health checks happen during the newborn window. The heel prick blood test, which screens for dozens of genetic and metabolic conditions, is performed when your baby is 24 to 48 hours old. Thirteen U.S. states require a second round of screening at one to two weeks. A hearing test is done any time after 12 hours of age, usually before you leave the hospital. If your baby doesn’t pass the hearing screen, a follow-up with a specialist is typically scheduled one to two weeks later.
Newborn vs. Infant vs. Baby
These terms overlap in everyday conversation, but they have distinct meanings in medical contexts. “Newborn” covers the first 28 days. “Infant” runs from birth through the first year. “Baby” has no formal medical definition and is used casually for children up to toddler age. So every newborn is an infant, but not every infant is still a newborn.
In practical terms, the shift from “newborn” to “infant” around one month coincides with real changes you’ll notice at home. By four to six weeks, most babies start having slightly longer stretches of wakefulness, become more visually alert, and begin producing social smiles. The intense, sleep-dominated cocoon of the first month gradually gives way to more interactive behavior.