A baby is considered premature if born before 37 weeks of pregnancy are completed. A full-term pregnancy lasts about 40 weeks, so any baby arriving more than three weeks early falls into the preterm category. That said, there’s a huge difference between a baby born at 36 weeks and one born at 25 weeks, which is why doctors break prematurity into distinct subcategories based on gestational age.
Categories of Premature Birth
The World Health Organization divides preterm birth into three groups:
- 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 vast majority of premature babies fall into the moderate to late preterm range. These babies generally face fewer complications than those born earlier, but they still need closer monitoring than full-term infants. Babies born extremely preterm require the most intensive medical support because their organs, especially the lungs and brain, have had the least time to develop.
Why Gestational Age Matters So Much
Every additional week in the womb makes a measurable difference in a baby’s readiness for life outside it. The lungs are one of the last organs to mature. Before about 34 weeks, babies often lack enough of a substance called surfactant, which keeps the tiny air sacs in the lungs from collapsing. Without it, breathing becomes extremely difficult. This is the core problem behind respiratory distress syndrome, the most common complication of premature birth.
The brain is also especially vulnerable. Fragile blood vessels in a premature baby’s brain can bleed, and the earlier the birth, the higher the risk. Bleeding in the brain can range from mild (resolving on its own) to severe, potentially causing long-term neurological effects. The immune system is similarly underdeveloped, leaving preterm babies more susceptible to serious infections.
The digestive system poses its own challenges. Premature babies can develop a dangerous intestinal condition where portions of the bowel wall become inflamed and begin to die. This is more common in the earliest preterm babies and requires close surveillance.
Survival at the Earliest Gestational Ages
Viability, the point at which a baby can survive outside the womb with medical help, has shifted earlier over the past two decades. Current U.S. data shows survival rates of 30% to 35% for babies born at 22 weeks who receive active resuscitation, up from about 23% a decade earlier. Some specialized centers report even higher numbers. One academic medical center reported 66% survival to discharge among babies resuscitated at 22 weeks over a 10-year period. There are even documented cases of survival at 21 weeks, though outcomes at that age remain highly uncertain.
By 28 weeks, survival rates climb above 90% in well-equipped hospitals. By 32 weeks, most babies survive with relatively few long-term complications, though they still benefit from extra medical support in the early days.
Late Preterm Babies Are Not “Almost Full Term”
Babies born between 34 and 36 weeks often look healthy and may be close to full-term size. This can create a false sense of security. Late preterm infants face real risks that full-term babies don’t.
Jaundice is significantly more common and more severe in this group. Their livers are still immature, making them slower to clear bilirubin, the yellow pigment that builds up in the blood after birth. Left untreated, severe jaundice can cause a rare but serious form of brain damage. Late preterm babies also struggle more with feeding, particularly breastfeeding, because their sucking reflex and coordination aren’t fully developed. They have more difficulty regulating body temperature and are more prone to low blood sugar and breathing problems in the first few days.
These babies may go home sooner than very preterm infants, but they need careful follow-up in the first week or two, especially around feeding and weight gain.
What Premature Babies Look Like
Premature babies look noticeably different from full-term newborns. Their skin is thin, translucent, and shiny, often with veins visible underneath. They tend to have very little body fat, giving them an angular appearance. Fine, soft hair called lanugo may cover their bodies. Their ears are soft and pliable, folding easily and not springing back the way a full-term baby’s ears do. The earlier the baby, the more pronounced these features are.
Risk Factors for Premature Birth
A previous preterm birth is the single strongest predictor of having another one. Carrying twins or triplets dramatically increases the odds as well. More than 50% of twin pregnancies result in preterm birth, compared with about 10% of singleton pregnancies.
Other medical risk factors include a short cervix, high blood pressure, diabetes (including gestational diabetes), infections during pregnancy such as urinary tract infections or bacterial vaginosis, and abnormalities of the uterus. Placenta previa, where the placenta covers the cervical opening, is another contributor. Women who’ve had a prior cesarean delivery or uterine surgery have a higher risk of uterine complications that can trigger early delivery.
Beyond medical conditions, age plays a role. Women younger than 18 and those older than 35 face higher rates of preterm birth. Getting pregnant again less than six months after a previous birth also raises risk. Smoking, alcohol use, and lack of prenatal care are all associated with higher rates of premature delivery. Racial disparities persist as well: Black and American Indian/Alaska Native women in the U.S. experience preterm birth at higher rates than white women.
What Happens Before a Preterm Birth
When doctors anticipate a premature delivery, one of the most effective interventions happens before the baby arrives. Steroid injections given to the mother speed up the baby’s lung development and reduce the severity of multiple complications. Babies whose mothers received this treatment have about a third lower risk of respiratory distress syndrome, roughly half the risk of brain bleeding, and a significantly lower risk of death compared with babies whose mothers did not receive it. These injections are recommended when preterm delivery is expected within seven days, typically for pregnancies between 24 and 37 weeks.
Levels of Newborn Care
Not all hospitals are equipped to care for all premature babies. Newborn care units are classified into levels based on what they can handle. A Level II special care nursery can care for babies born at 32 weeks or later who are moderately ill with problems expected to resolve quickly. These units can provide short-term breathing support but will transfer sicker or smaller babies to a higher-level facility.
Level III NICUs provide comprehensive intensive care for babies born before 32 weeks or at any gestational age with critical illness. They have around-the-clock access to pediatric specialists, advanced imaging, and a full range of breathing support. Level IV regional NICUs offer everything a Level III does, plus the ability to perform complex surgeries on-site. If your hospital doesn’t have the right level of care, your baby will be stabilized and transported to one that does.
Corrected Age and Development
Premature babies don’t follow the same developmental timeline as full-term babies, at least not at first. Pediatricians use something called “corrected age” to set realistic expectations. You calculate it by subtracting the number of weeks your baby was born early from their actual age. A 6-month-old who was born at 28 weeks (12 weeks early) would have a corrected age of 3 months. Developmental milestones like rolling over, sitting up, and babbling are measured against this corrected age, not the calendar age.
The American Academy of Pediatrics recommends using corrected age until a child reaches 2 years old. After that, most premature children have caught up enough developmentally that the adjustment is no longer necessary. Some children, particularly those born extremely preterm, may continue to need developmental support beyond that point, but the corrected age framework gives families a much more accurate picture during those critical early months.