A baby born before 37 weeks of pregnancy is considered premature. A full pregnancy lasts about 40 weeks, so any delivery three or more weeks early falls into the preterm category. But not all premature births carry the same risks. The earlier a baby arrives, the more medical support they typically need, and doctors use specific categories to describe how early a birth occurs.
Categories of Preterm Birth
The World Health Organization breaks preterm birth into three groups based on gestational age:
- Extremely preterm: born before 28 weeks
- Very preterm: born between 28 and 31 weeks
- Moderate to late preterm: born between 32 and 36 weeks
The moderate to late preterm group accounts for the vast majority of premature births. These babies generally do well, though they still face higher risks than babies born at 39 or 40 weeks. Babies born extremely preterm, before 28 weeks, face the steepest challenges because their organs are still in early stages of development.
Where “Term” Actually Begins
Reaching 37 weeks does not mean a baby is fully term in the way doctors once thought. The American College of Obstetricians and Gynecologists redefined the categories in 2013 to reflect what the data actually shows about newborn health:
- Early term: 37 weeks 0 days through 38 weeks 6 days
- Full term: 39 weeks 0 days through 40 weeks 6 days
- Late term: 41 weeks 0 days through 41 weeks 6 days
- Postterm: 42 weeks and beyond
This distinction matters because babies born at 37 or 38 weeks, while not premature, still have slightly higher rates of breathing problems and feeding difficulties compared to those born at 39 or 40 weeks. The “full term” sweet spot starts at 39 weeks.
The Limit of Viability
The earliest point at which a baby can potentially survive outside the womb falls between 22 and 25 weeks, a window doctors call periviability. Survival at these ages depends heavily on the hospital’s resources, the baby’s weight, and whether the NICU team provides active intervention.
A study published in Pediatrics looking at infants born between 2020 and 2022 found that survival ranged from about 25% at 22 weeks to 82% at 25 weeks. Among babies who received active life support after birth, survival at 22 weeks rose to about 35%. Even among survivors at these very early ages, few escaped serious complications, and hospital stays were long.
A baby born before 26 weeks or weighing less than about 1.5 pounds is often called a micro-preemie. These infants require the most intensive medical support and face the highest risk of long-term health effects.
Why Earlier Weeks Are Riskier
The biggest factor in preterm complications is lung development. The lungs begin producing surfactant, a substance that keeps the air sacs open so a baby can breathe, at around 24 weeks. But they don’t produce enough of it to support reliable breathing until about 32 weeks. This is why respiratory distress syndrome is the most common serious problem in premature infants. The risk drops dramatically with each passing week: roughly 60% of babies born before 24 weeks develop it, compared to less than 5% of those born after 34 weeks.
Beyond the lungs, premature babies can struggle with temperature regulation because they lack the body fat needed to stay warm. Their digestive systems may not be ready to coordinate sucking and swallowing for feeding. Blood sugar regulation, immune function, and brain development are all still works in progress, with the degree of vulnerability tied directly to how many weeks early the birth occurs.
Risks for Late Preterm Babies (34 to 36 Weeks)
Parents sometimes hear “34 weeks” or “35 weeks” and assume their baby will be essentially fine. Most of these babies do well, but they’re not out of the woods the way a 39-week baby would be. Compared to babies born between 37 and 40 weeks, a baby born at 34 weeks is roughly 10 times more likely to need help breathing and about 9 times more likely to need a ventilator for more than six hours. At 35 weeks, the odds of NICU admission are still about 11 times higher than for a full-term baby.
Common issues in late preterm infants include low blood sugar, jaundice, difficulty maintaining body temperature, and trouble feeding. These problems are usually manageable and temporary, but they can mean a longer hospital stay and extra monitoring during the first days and weeks of life.
How Long Premature Babies Stay in the Hospital
A useful rule of thumb: most premature babies stay in the NICU until around their original due date, though babies born after 30 weeks often go home several weeks earlier than that. The more premature the birth, the longer the stay.
Research tracking very preterm infants found these median hospital stays:
- Born at 24 weeks: about 123 days (discharged around 41.5 weeks corrected age)
- Born at 26 weeks: about 92 days
- Born at 28 weeks: about 66 days
- Born at 30 weeks: about 42 days
- Born at 31 weeks: about 34 days
Babies born at 24 or 25 weeks tend to stay slightly past their due date before discharge. As gestational age increases, babies are generally ready to go home sooner relative to their due date. A baby born at 30 weeks, for example, typically goes home about 30 days before the date they were originally expected.
Discharge depends on a few key milestones: the baby needs to breathe on their own, maintain their body temperature in an open crib, and feed well enough to gain weight consistently. These milestones can come at slightly different times for each baby, so the ranges above are averages rather than guarantees.