The newborn stage lasts 4 weeks, or the first 28 days of life. This is the formal medical definition used by the World Health Organization, which refers to a baby in this period as a “neonate.” While 4 weeks is the clinical cutoff, many pediatricians and parents think of the broader adjustment period as lasting closer to 12 weeks, sometimes called the “fourth trimester.”
Why 28 Days Is the Medical Cutoff
The first 28 days are singled out because they represent the highest-risk window in a child’s first year. A baby’s body is still adapting to life outside the womb: regulating temperature, learning to feed, and transitioning to breathing air full-time. Most of the standard medical screenings happen in this window, and pediatricians monitor growth and feeding especially closely during these weeks.
After 28 days, your baby technically moves into the “infant” stage, which lasts until their first birthday. But the shift isn’t dramatic. A 5-week-old still looks and acts a lot like a 3-week-old. The 28-day mark is a medical boundary, not a developmental cliff.
What Happens in Those 4 Weeks
Weight Changes
Most newborns lose weight in the first few days after birth. This is normal. They typically start regaining weight between days 3 and 5, and about 80 percent of babies are back to their birth weight by 2 weeks old. A weight loss of 10 percent or more from birth weight warrants a closer look from your pediatrician, usually to evaluate feeding.
Feeding
Newborns eat frequently. Breastfed babies feed about 8 to 12 times in 24 hours, which works out to roughly every 2 to 4 hours. In the very first days, feedings can come as often as every 1 to 3 hours. There’s no fixed ounce target for breastfed babies; they generally take what they need and stop when full. Formula-fed babies follow a similar schedule but with more predictable volumes per feeding.
Sleep
Newborns sleep a lot, around 16 to 17 hours per day, but in frustratingly short stretches. Most sleep only 1 to 2 hours at a time, day and night. They don’t develop regular sleep cycles until about 6 months of age, so the first 4 weeks are particularly unpredictable. Your baby has no concept of daytime versus nighttime yet.
Movement and Reflexes
By the end of the newborn stage, your baby will be noticeably more alert and responsive than they were at birth. In the first 4 weeks, movement is mostly jerky and reflexive: quivering arm thrusts, tight fists, and strong reflex reactions. They’ll start bringing their hands within range of their eyes and mouth and can turn their head side to side while on their stomach, though their head still flops backward without support. Coordination is minimal but visibly improving week to week.
Senses
Newborns can focus on objects 8 to 12 inches away, roughly the distance to your face during feeding. They prefer high-contrast patterns and human faces over anything else. Their eyes may wander or occasionally cross, which is normal at this age. Hearing is fully developed at birth. Your baby can recognize and react to loud sounds and may turn toward familiar voices. They also have a strong sense of smell, preferring sweet scents and already recognizing the smell of their own mother’s breast milk. They prefer soft textures and gentle handling.
Screenings During the Newborn Stage
Three standard screenings happen in the first days of life, usually before you leave the hospital. The blood spot screening (often called the “heel stick”) uses a few drops of blood from your baby’s heel to check for dozens of serious but treatable conditions. This happens between 24 and 48 hours after birth. Pulse oximetry screening happens in the same window: a small sensor wraps around your baby’s wrist and foot to measure oxygen levels and detect certain heart defects. Hearing screening can happen any time after your baby is 12 hours old and involves soft sounds played through tiny earbuds or headphones while a computer measures the response.
Your baby will also typically have their first pediatric visit within one week of birth, with follow-ups over the first several weeks to track weight gain and overall health.
The Fourth Trimester: A Broader View
While the newborn stage is technically 4 weeks, many experts frame the first 12 weeks after birth as the “fourth trimester.” The idea is that human babies are born relatively underdeveloped compared to other mammals, and the first 3 months of life are essentially an extension of pregnancy. During this time, your baby still craves the conditions of the womb: warmth, closeness, gentle motion, and consistent feeding.
The fourth trimester concept also applies to the birthing parent. The period between birth and 12 weeks postpartum involves significant physical recovery and emotional adjustment. Pediatric care is intensive during this window, with frequent check-ins on both the baby and the parent. So while your baby stops being a “newborn” at 4 weeks by medical standards, the intensity of early parenthood and your baby’s dependency stays high for a full 3 months.
Week-by-Week Changes to Expect
Week 1 is the most physically dramatic. Your baby may lose weight, their umbilical cord stump is still attached, and feedings are frequent and sometimes difficult to establish. Sleep is erratic, and your baby spends most of their awake time eating.
By week 2, most babies are starting to regain weight. Feedings may settle into a slightly more predictable rhythm, though “predictable” is relative. Your baby is still sleeping in short bursts around the clock.
Weeks 3 and 4 bring the first hints of emerging personality. Your baby becomes more alert during waking periods, may start tracking your face briefly with their eyes, and their movements grow slightly smoother. They’re still entirely dependent on you, but the shift from the sleepy, curled-up newborn of week 1 is noticeable. By the end of week 4, you’re past the official newborn stage and into the longer stretch of infancy, though day-to-day life won’t feel much different for a while yet.