A baby born before 37 weeks of gestation is considered premature. A full-term pregnancy runs 39 weeks through 40 weeks and 6 days, so even births at 37 or 38 weeks are classified as “early term” rather than truly full term. The earlier a baby arrives before that 37-week threshold, the higher the risks, which is why doctors break prematurity into distinct categories.
Categories of Preterm Birth
The World Health Organization divides preterm birth into three tiers based on gestational age:
- Extremely preterm: less than 28 weeks
- Very preterm: 28 to less than 32 weeks
- Moderate to late preterm: 32 to 37 weeks
Most premature births fall into the moderate-to-late preterm range. These babies generally do well, but they still face measurably higher risks than infants who reach full term. The distinctions matter because each category carries different expected challenges, hospital stays, and long-term outlooks.
Why 37 Weeks Is the Cutoff
The 37-week line isn’t arbitrary. The lungs are among the last organs to fully mature, and they typically aren’t ready until around 37 weeks. Before that point, a baby may not produce enough surfactant, a substance that keeps the tiny air sacs in the lungs from collapsing with each breath. This is the single biggest reason premature infants struggle with breathing and often need support in a neonatal intensive care unit.
If preterm delivery looks likely, doctors can give steroid injections to the mother to help speed up the baby’s lung development. This is recommended for pregnancies between 24 and 34 weeks when delivery appears imminent, and it can also be considered as early as 23 weeks or as late as 36 weeks depending on the situation. Those steroids can make a significant difference in a baby’s ability to breathe at birth.
Early Term Is Not the Same as Full Term
The American College of Obstetricians and Gynecologists uses a precise set of definitions that many people aren’t aware of:
- 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
- Post-term: 42 weeks and beyond
This distinction matters more than it might seem. A large national cohort study published in the BMJ found that babies born at 37 to 38 weeks had a higher risk of cognitive impairment compared with those born at 39 to 40 weeks. The risk isn’t dramatic, but it’s real enough that medical guidelines now discourage elective deliveries before 39 weeks unless there’s a medical reason.
Survival at the Earliest Weeks
For families facing an extremely early delivery, gestational age is the strongest predictor of survival. At 22 and 23 weeks, survival to hospital discharge remains rare or zero in many medical centers. At 25 weeks, roughly 29% of live-born infants survive to discharge. That number jumps sharply over the next two weeks: about 61% at 26 weeks and 82% at 27 weeks. These figures come from a recent study in Frontiers in Pediatrics and reflect outcomes at a single center, so results vary by hospital resources and regional care standards. High-income countries with advanced neonatal units generally report higher survival rates at the earliest gestational ages.
The threshold of viability, the earliest point at which survival outside the womb is possible, is generally considered to be around 22 to 24 weeks. But “possible” and “likely” are very different things at those ages, and families facing delivery this early are typically counseled about both survival odds and the likelihood of long-term complications.
What Premature Babies Typically Need
The challenges a premature baby faces depend heavily on how early they arrive. Across all categories, the most common needs include help with breathing, feeding, gaining weight, and maintaining body temperature. A baby born at 35 weeks might need a few days of monitoring and feeding support. A baby born at 28 weeks could spend weeks or months in intensive care.
Breathing difficulties are the hallmark concern. Underdeveloped lungs can lead to respiratory distress syndrome, a condition where the lungs can’t stay inflated properly. Some premature babies also experience apnea, temporary pauses in breathing that require monitoring. Feeding is another persistent challenge because the coordination needed to suck, swallow, and breathe simultaneously doesn’t fully develop until around 34 to 36 weeks. Many earlier preterm infants receive nutrition through a tube until they can feed on their own.
Size at Birth by Gestational Age
Premature babies are smaller than full-term newborns, sometimes dramatically so. At 28 weeks, a typical baby weighs around 1,100 to 1,175 grams (roughly 2.4 to 2.6 pounds). By 32 weeks, that climbs to about 1,800 to 1,900 grams (4.0 to 4.2 pounds). At 36 weeks, the average is around 2,750 to 2,850 grams (6.0 to 6.3 pounds), which is still about a pound lighter than the average full-term baby. Girls tend to weigh slightly less than boys at every gestational age.
Weight alone doesn’t determine outcomes, but it gives doctors a quick reference point for what to expect and how to plan care. A baby who is small for their gestational age (lighter than expected) may face additional challenges beyond those tied to prematurity itself.
Long-Term Developmental Risks
Most late preterm babies catch up to their peers developmentally within the first few years. But the risks aren’t zero. A national cohort study tracking children born between 1998 and 2012 found that babies born at 32 to 33 weeks had a 4.75% higher absolute risk of neurodevelopmental impairment compared with those born at 39 to 40 weeks. For babies born at 34 to 36 weeks, the increased risk was about 2%. These impairments include delays in motor skills, language, and cognitive development.
The pattern is consistent: risk declines gradually with each additional week in the womb, all the way through 41 weeks. Even a week or two can matter. This gradient is one reason doctors try to delay preterm labor when safely possible, because every additional day of development in utero improves the odds.
For extremely preterm babies, the long-term picture is more complex. Infants born before 28 weeks face higher rates of cerebral palsy, vision and hearing problems, and learning disabilities. Many do well, but ongoing developmental monitoring through early childhood is standard practice for this group.