The newborn stage lasts 28 days. The World Health Organization defines a neonate as a baby in the first 28 days of life, making this one of the shortest and most rapid developmental periods a human will ever experience. But while the medical definition draws a clean line at four weeks, the physical and behavioral transition out of the newborn stage is more gradual, with some hallmark newborn traits fading within days and others lingering for months.
What Happens During the First 28 Days
The newborn period is defined by a body adjusting to life outside the womb. In the first week, babies typically lose up to 7% of their birth weight as they learn to feed and their bodies shed excess fluid. Most regain that weight by day 10. Feeding happens in tiny amounts at first: a newborn’s stomach at birth holds only about 1 to 2 teaspoons of milk, roughly the size of a marble. By day 10, it grows to the size of a ping-pong ball, holding around 2 ounces per feeding.
Sleep dominates these early weeks. Newborns sleep 16 to 17 hours per day, but only 1 or 2 hours at a stretch. They don’t develop regular sleep cycles until around 6 months, which is why the first month feels so fragmented for parents. Movements are jerky and involuntary, driven by primitive reflexes rather than intentional control.
The umbilical cord stump, one of the most visible signs that your baby is brand new, typically falls off 1 to 3 weeks after birth. If it hasn’t separated by three weeks, that’s worth mentioning to your pediatrician, as it can occasionally signal an infection or immune system issue.
Jaundice and Other Early Changes
Mild jaundice, a yellowish tint to the skin, is one of the most common things parents notice in the first week. Physiologic jaundice in full-term infants typically appears about 24 hours after birth, peaks between 48 and 96 hours, and resolves on its own by 2 to 3 weeks. Hospitals screen bilirubin levels before discharge to make sure the jaundice falls within a normal range. For most babies, it clears without any intervention during the newborn period itself.
What Newborns Can See and Hear
Newborn vision is limited but not as blank as many parents assume. Contrary to the popular idea that babies see only in black and white, even neonates can detect some color. Their color vision is poor, though: colors need to be large, bold, and highly saturated to register. In one study, more than 75% of newborns oriented toward a large patch of bright red on a grey background, while over 80% failed to notice blue under the same conditions. Newborns focus best on objects about 8 to 12 inches from their face, roughly the distance to a parent’s face during feeding.
Hearing develops faster. By around one month, babies start recognizing familiar sounds and may turn their head toward a parent’s voice.
Reflexes That Define the Newborn Period
Newborns come equipped with a set of primitive reflexes, automatic motor responses controlled by the brainstem rather than conscious thought. These are some of the clearest biological markers of the newborn stage, and they fade on different timelines as the brain matures.
- Rooting reflex: When you stroke a newborn’s cheek, they turn toward the touch and open their mouth. This helps them find the breast or bottle. It begins to fade after the first month.
- Moro (startle) reflex: A sudden noise or movement causes the baby to throw their arms out and then pull them back in. This persists until about 6 months.
- Grasping reflex: Place a finger in a newborn’s palm and they’ll grip it tightly. This also lasts until around 6 months.
- Asymmetric tonic neck reflex: When a baby’s head turns to one side, the arm on that side extends while the opposite arm bends, like a fencing pose. This fades by about 3 months.
These reflexes are gradually replaced by voluntary, intentional movements between 4 and 6 months of age. Their presence in the early weeks, and their disappearance on schedule, is one way pediatricians assess healthy neurological development.
How the Transition to Infancy Looks
The shift from “newborn” to “young infant” isn’t a single moment. It’s a cluster of changes that unfold over the second and third months of life, well past the 28-day medical cutoff. Here’s what that progression typically looks like.
At around 2 months, babies begin to control their head when held upright, and their movements become less jerky and more purposeful. This is also when most babies start to smile in response to another person’s smile, a social milestone that feels like a turning point for many parents. Cooing and repeating vowel sounds begin around this time too.
By 3 months, babies can lift their head and chest while lying on their tummy (supported on their elbows), grab toys and bring them to their mouth, distinguish between colors, make eye contact reliably, and experiment with sounds like squeals, growls, and raspberries. They may imitate sounds and respond to familiar voices with visible excitement. These are all signs that the brain’s higher-level circuits are taking over from the primitive reflexes that dominated the newborn weeks.
The Medical vs. Practical Newborn Stage
Parents often use “newborn” more loosely than doctors do. Medically, the newborn period is a precise 28-day window, chosen because this is when babies face the highest risk of certain complications and need the most frequent monitoring. Practically, many parents think of the “newborn phase” as lasting closer to 2 or 3 months, because that’s how long it takes for sleep to become slightly less chaotic, for the baby to become more interactive, and for feeding to feel more established.
Both framings are useful. The 28-day definition matters for medical care and screening schedules. The broader 3-month window better reflects the lived experience of caring for a very young baby and watching them gradually become more alert, responsive, and physically capable. Most clothing brands size “newborn” for roughly the first 4 weeks and up to about 8 pounds, which aligns more closely with the medical definition.