A preemie is a baby born before 37 weeks of pregnancy. A full-term pregnancy lasts about 40 weeks, so a preemie arrives at least three weeks early. The term is an informal shortening of “premature infant,” and it’s used widely by parents, nurses, and doctors alike. How early a baby arrives matters enormously, though. A baby born at 35 weeks faces a very different situation than one born at 26 weeks.
Categories of Prematurity
Not all preemies are the same. The World Health Organization breaks preterm birth into three categories based on how early the baby is born:
- Moderate to late preterm: 32 to 37 weeks. This is the most common group. Many of these babies need only brief medical support.
- Very preterm: 28 to less than 32 weeks. These babies typically need weeks of specialized hospital care.
- Extremely preterm: less than 28 weeks. These are the smallest and most vulnerable, often weighing just over a pound.
The vast majority of preemies fall into the moderate to late preterm category. Babies born closer to 37 weeks may look and act much like full-term newborns, while those born before 28 weeks face serious medical challenges that require months of intensive care.
What a Preemie Looks Like
Premature babies look noticeably different from full-term newborns, especially the earlier they arrive. Their skin is thin, shiny, and pink, sometimes almost translucent, with veins clearly visible beneath the surface. They have very little body fat, which makes their limbs appear fragile and angular. Many are covered in fine, downy hair called lanugo that full-term babies typically shed before birth. Their heads may seem disproportionately large compared to their bodies. The earlier the birth, the more pronounced these features tend to be.
Why Some Babies Come Early
In many cases, doctors simply don’t know what triggers early labor. But several factors raise the likelihood. Carrying twins, triplets, or more is one of the strongest risk factors, partly because the uterus stretches more and partly because multiple pregnancies carry higher rates of complications. High blood pressure conditions during pregnancy, particularly preeclampsia, are another common cause. Infections, including COVID-19, also increase the risk.
Women who have already had one preterm birth are more likely to have another. And those who conceive through IVF or other assisted reproductive technologies face higher preterm rates, primarily because these methods are more likely to result in multiple pregnancies.
Survival Rates by Gestational Age
Gestational age is the single biggest predictor of a preemie’s outcome. Before 24 weeks, survival rates are generally below 50 percent, and many hospitals consider this the threshold of viability. At 24 weeks, survival improves to roughly 60 to 70 percent. By 28 weeks, 80 to 90 percent of babies survive. After 32 weeks, the odds are overwhelmingly in the baby’s favor, and most of these infants go on to live healthy lives.
Each additional week in the womb makes a measurable difference. Even a few extra days can allow the lungs, brain, and digestive system to mature in ways that dramatically reduce complications.
What Happens in the NICU
Most preemies spend time in a neonatal intensive care unit, or NICU. How long depends on how early they were born. A baby born at 34 weeks might go home in a week or two. One born at 26 weeks could stay for three months or longer.
The first priority is temperature. Preemies don’t have enough body fat to regulate their own heat, so they’re placed in an incubator, an enclosed, temperature-controlled bassinet. Sensors taped to the skin continuously monitor heart rate, breathing, blood pressure, and temperature.
Breathing support is common. The lungs are one of the last organs to fully mature, so many preemies need help getting enough oxygen. Some use a ventilator, while others get continuous positive airway pressure (CPAP), a gentler system that keeps the airways open. Babies whose lungs are very underdeveloped may receive a medication called surfactant, a substance that coats the inside of the lungs and prevents them from collapsing with each breath.
Feeding is another challenge. Preemies born very early can’t coordinate the sucking and swallowing needed for breastfeeding or bottle-feeding. At first, they receive fluids and nutrients through a vein. As they mature, breast milk or specialized preterm formula is delivered through a thin tube that passes through the nose into the stomach. The transition to feeding by mouth is one of the final milestones before a baby can go home.
Corrected Age and Development
Preemies hit developmental milestones on a different timeline than full-term babies, and pediatricians account for this using something called corrected age. The idea is straightforward: you subtract the number of weeks a baby was born early from their actual age. A baby who is 16 weeks old but was born 10 weeks early has a corrected age of about 6 weeks. That means their doctor would expect them to behave more like a 6-week-old than a 4-month-old.
This adjustment is typically used until a child turns two, at which point most preemies have caught up to their peers developmentally. It’s a useful tool for parents who might otherwise worry that their baby is falling behind. A preemie who isn’t rolling over at four months may be right on track when you consider their corrected age.
Some extremely preterm babies do face longer-term developmental differences, including challenges with motor skills, learning, or vision. Early intervention programs, which provide physical therapy, speech therapy, and developmental support, can make a significant difference for these children during the first few years of life.