The neonatal period is the first 28 days of a baby’s life, starting from the moment of birth. It’s the most rapid and dramatic phase of human adaptation, as a newborn transitions from depending entirely on the placenta to breathing, eating, and regulating body temperature independently. This period is divided into two sub-phases: the early neonatal period (birth through day 6) and the late neonatal period (days 7 through 27).
What Happens to the Body at Birth
The shift from womb to outside world requires nearly every organ system to change how it works, most of it within minutes. Before birth, a baby’s lungs are filled with fluid rather than air, and oxygen comes from the placenta through the umbilical cord. With the first breath, resistance in the blood vessels of the lungs drops sharply, allowing blood to flow there for the first time in large quantities. At the same time, clamping the umbilical cord disconnects the placenta’s low-resistance blood supply, which forces blood pressure in the baby’s own circulation to rise.
These two pressure changes work together to close special openings in the fetal heart. A small flap between the upper chambers of the heart (called the foramen ovale) seals shut because pressure on the left side now exceeds the right. Another vessel that bypassed the lungs during fetal life, the ductus arteriosus, constricts in response to the higher oxygen levels in the blood. Within about 10 minutes of birth, blood flow reverses through this vessel, sending more blood to the lungs. In the first hour of life, the heart’s pumping output roughly doubles.
The lungs undergo their own transformation. Labor itself triggers the release of surfactant, a substance that keeps the tiny air sacs from collapsing. Once breathing starts, stretching of those air sacs increases surfactant production further. Meanwhile, the lung tissue switches from actively secreting fluid to actively absorbing it. Most of the fetal lung fluid clears within about two hours of birth.
Weight Changes in the First Weeks
Almost all newborns lose weight in the first few days. A loss of 7 to 10 percent of birth weight is typical and happens primarily because the baby sheds excess water that accumulated in tissues before birth. For a baby born at 7.5 pounds, that translates to roughly half a pound to three-quarters of a pound. Full-term babies generally regain their birth weight within 7 to 10 days. Premature infants take longer, typically 10 to 15 days, and extremely small preemies may need even more time.
This early weight loss is normal, but tracking it matters. A loss significantly beyond 10 percent, or failure to start gaining by the end of the first week, can signal feeding difficulties that need attention.
Sleep During the Neonatal Period
Newborns sleep roughly 16 hours a day, split fairly evenly between daytime and nighttime, about 8 hours each. There’s no real day-night rhythm yet. Sleep comes in short bursts because the baby’s stomach is small and hunger wakes them frequently.
About half of a newborn’s sleep is REM (rapid eye movement) sleep, the lighter stage associated with dreaming in older children and adults. This is a much higher proportion of REM than at any other point in life. The remaining sleep cycles through four stages of progressively deeper non-REM sleep. During the deepest stages, the baby is quiet and still. During REM, you’ll notice twitching, irregular breathing, and eye movement under the lids. These patterns gradually shift over the first few months as the brain matures.
Reflexes and Neurological Development
Babies are born with a set of involuntary reflexes controlled by the brainstem, the most primitive part of the brain. These reflexes serve as both survival tools and useful markers of neurological health. They include:
- Rooting reflex: stroking the cheek causes the baby to turn toward the touch with an open mouth, helping them find the breast or bottle. This reflex begins to fade after about one month.
- Sucking reflex: anything that touches the roof of the mouth triggers rhythmic sucking.
- Moro reflex: a sudden sensation of falling causes the baby to throw their arms out and then pull them back in. This startle response disappears by about six months.
- Grasp reflex: pressing on a newborn’s palm causes their fingers to curl tightly around the object. Also disappears around six months.
- Asymmetric tonic neck reflex: turning the baby’s head to one side causes them to extend the arm on that side while flexing the opposite arm, sometimes called the “fencing” position. This fades by about three months.
The absence of these reflexes during the neonatal period, or their persistence well past 4 to 6 months of age, can be an early indicator of conditions like cerebral palsy.
Routine Screenings in the First Days
Within minutes of birth, a baby receives an Apgar assessment. This quick check evaluates five things: breathing effort, heart rate, muscle tone, reflexes, and skin color. Each is scored from 0 to 2, giving a total out of 10. A score of 7 or above is considered normal. A perfect 10 is rare because most newborns lose a point for bluish hands and feet, which is expected in the first minutes of life. The Apgar is performed at 1 minute and again at 5 minutes after birth.
Before leaving the hospital, newborns go through three standard screenings. A blood spot test (sometimes still called the “PKU test,” though it now checks for far more than one condition) screens for a range of serious metabolic and genetic disorders. Each state maintains its own list of conditions it tests for. A pulse oximetry screening clips a small sensor to the baby’s skin to detect certain critical congenital heart defects by measuring oxygen levels. A hearing screening uses tiny earbuds or earphones to check for deafness or hearing loss. All three are quick, painless, and can catch conditions that aren’t visible but benefit enormously from early treatment.
Warning Signs During the Neonatal Period
The neonatal period carries the highest vulnerability of any stage in childhood. The World Health Organization identifies several danger signs that warrant immediate medical attention: difficulty feeding or refusal to feed, reduced activity or unusual lethargy, labored or rapid breathing, fever, convulsions or seizures, and cold skin. Many of these overlap, since a baby who is becoming seriously ill often shows feeding problems and low energy first. Because newborns can deteriorate quickly, any combination of these signs in the first 28 days is treated with urgency.
Why the 28-Day Cutoff Matters
The neonatal period is defined as 28 days because this window captures the time when a baby is most physiologically unstable. The cardiovascular system is still finalizing the closure of fetal heart structures. The immune system is almost entirely dependent on antibodies passed from the mother during pregnancy and through breast milk. Temperature regulation is immature, and the baby loses heat quickly relative to body size. By the end of the fourth week, most of these systems have stabilized enough that the baby enters a distinctly different phase of infancy, with new developmental milestones, longer sleep stretches, and a more robust ability to maintain basic body functions independently.