The newborn stage lasts 4 weeks (28 days) from the date of birth. In medical terms, this period is called the neonatal period, and it covers the most rapid and dramatic physical changes your baby will go through in their entire life. Understanding what happens during these four weeks helps you recognize what’s normal and what to expect as your baby adjusts to life outside the womb.
The Medical Definition: Birth to 28 Days
A neonate is a baby who is 4 weeks old or younger. After day 28, your baby is still an infant, but they’re no longer technically a newborn. This distinction matters in pediatric care because the first 28 days carry unique health considerations, screening schedules, and feeding challenges that differ from the broader infant stage, which extends to 12 months.
Weight Changes in the First Weeks
One of the most anxiety-inducing parts of the newborn stage is watching your baby lose weight right after birth. This is completely normal. Babies hit their lowest weight between 2 and 3 days after birth, regardless of whether they were delivered vaginally or by cesarean section.
How much they lose depends partly on how they’re fed. Exclusively breastfed babies typically lose between 5.5% and 8.6% of their birth weight, while formula-fed babies lose a bit less, ranging from about 2.4% to 7.5%. Losses up to around 10% are still within the expected range for most babies, though anything above that warrants a closer look from your pediatrician.
Most babies regain their birth weight by 3 weeks of age. Formula-fed infants tend to get there a little faster, around 16 to 17 days, while exclusively breastfed babies may take up to 21 days. So for nearly the entire newborn stage, your baby is working just to get back to where they started.
Feeding Around the Clock
Newborns eat constantly. In the first few days, your baby may want to feed every 1 to 3 hours. As the weeks progress, breastfed babies settle into a pattern of roughly 8 to 12 feedings in 24 hours, or about every 2 to 4 hours. There’s no long break overnight during this stage. Your baby’s stomach is tiny, empties quickly, and their brain hasn’t yet developed the ability to consolidate sleep into longer stretches.
Sleep Patterns: Lots of Hours, Little Structure
Newborns sleep about 16 hours a day, split roughly in half between daytime and nighttime. That sounds like a lot, but it comes in short, unpredictable bursts rather than long blocks. About half of a newborn’s sleep is spent in REM (the lighter, dream-heavy stage), which is why they startle, twitch, and wake so easily. Their sleep cycles are much shorter than an adult’s, and they cycle through stages of light and deep sleep multiple times before waking to feed again. A predictable sleep schedule won’t emerge until well after the newborn stage ends.
Physical Changes to Expect
Your baby’s body goes through visible transformations during these 28 days. Some babies are born with a fine layer of soft body hair called lanugo, especially if they arrived a bit early. This hair disappears within the first two months of life, often within just a few weeks.
The umbilical cord stump is another hallmark of the newborn period. It typically falls off 1 to 3 weeks after birth, with two weeks being the most common timeline. If it’s still attached after three weeks, it’s worth mentioning to your baby’s doctor.
Jaundice, the yellowish tint to the skin caused by the liver adjusting to its new workload, is extremely common. It typically appears within the first 24 hours, peaks between 48 and 96 hours (days 2 through 4), and resolves on its own by 2 to 3 weeks. Mild jaundice requires nothing more than monitoring, though higher levels may need treatment with light therapy.
Reflexes That Define the Newborn Period
Newborns arrive with a set of automatic reflexes that serve as early signs of a healthy nervous system. The rooting reflex, where your baby turns toward anything that touches their cheek, is strongest in the first month and starts to fade after that. The Moro reflex (the startled “arms out” reaction) and the grasping reflex both stick around longer, typically disappearing by six months. The asymmetric tonic neck reflex, where your baby extends one arm when their head turns to that side, fades by about three months.
These reflexes are involuntary. Over the first 4 to 6 months, the brain gradually matures enough to replace them with intentional, voluntary movements. Their presence during the newborn stage, and their disappearance on schedule, is one of the things your pediatrician checks at early visits.
How the Timeline Shifts for Premature Babies
If your baby was born before 37 weeks, the newborn stage gets a little more complicated. Pediatricians use something called “corrected age” to assess a preemie’s growth and development. This means subtracting the number of weeks your baby was born early from their actual age. A baby born at 34 weeks, for example, would be considered developmentally equivalent to a newborn even at 6 weeks of chronological age.
For very premature babies, corrected age is used for all growth assessments through at least 2 years, and some evidence supports using it through age 3. This adjustment ensures preemies aren’t unfairly compared to full-term babies who had more time to develop before birth. In practical terms, it means the “newborn-like” behaviors and needs of a premature baby can extend well beyond the standard 28-day window.
What Changes After 4 Weeks
The shift from newborn to older infant isn’t a dramatic overnight change. But by the end of the fourth week, a few things are different. Your baby has likely regained their birth weight and started gaining steadily. Jaundice has cleared. The cord stump is gone. Feeding, while still frequent, is more established, and you’re starting to recognize your baby’s hunger cues more reliably. Sleep is still fragmented, but you may notice slightly longer stretches at night starting to emerge in the weeks ahead.
The newborn stage is intense precisely because so many systems are coming online for the first time. Your baby’s liver, digestive system, temperature regulation, and nervous system are all calibrating simultaneously. By 28 days, the most acute adjustments are behind them, even though infancy still has a long way to go.