A premie (also spelled “preemie”) is a baby born before 37 weeks of pregnancy. A full-term pregnancy lasts about 40 weeks, so a premie arrives at least three weeks early, often before their organs have fully matured. Premature birth is common, and outcomes vary widely depending on how early the baby is born.
Categories of Prematurity
Not all premies are the same. The earlier a baby arrives, the more medical support they typically need. The World Health Organization breaks preterm birth into three categories based on gestational age:
- Moderate to late preterm (32 to 37 weeks): The most common group. These babies often need some extra monitoring but generally do well.
- Very preterm (28 to less than 32 weeks): These babies usually require weeks or months of specialized hospital care.
- Extremely preterm (less than 28 weeks): The smallest and most vulnerable group, facing the highest risk of serious complications.
Most premature births fall into the moderate to late preterm range. Babies born closer to 37 weeks may look and act almost like full-term newborns, while those born at 24 or 25 weeks can weigh barely over a pound and fit in the palm of a hand.
What a Premie Looks Like
Premature babies look noticeably different from full-term newborns. They have very little body fat, which makes their skin appear thin and pink, with veins visible underneath. Their ears are soft and don’t spring back when folded, because the cartilage hasn’t fully developed yet. They have scant hair on their head, though their body may be covered in fine, downy hair called lanugo, which full-term babies shed before birth. The earlier the baby is born, the more pronounced these features tend to be.
Why Premature Birth Happens
Sometimes premature labor starts on its own with no clear explanation. But several known risk factors raise the likelihood. Carrying twins, triplets, or more is one of the strongest predictors. Women who have delivered preterm before are at significantly higher risk of it happening again.
Medical conditions during pregnancy also play a role: high blood pressure, diabetes, certain infections (including urinary tract and vaginal infections), and problems with the placenta or cervix can all trigger early delivery. A short cervix, or one that shortens too early in pregnancy, is a specific structural risk factor.
Beyond medical causes, age matters. Women younger than 18 and older than 35 face higher rates of preterm birth. Lifestyle factors like smoking, alcohol use, high stress, and having less than six months between pregnancies also increase risk. Racial disparities exist as well: infants born to Black mothers in the United States are more likely to arrive preterm than those born to white mothers.
Health Challenges Premies Face
The core problem for most premies is that their organs haven’t had enough time to finish developing. The lungs are the most immediate concern. Premature lungs often can’t produce enough surfactant, a slippery substance that keeps the tiny air sacs in the lungs from collapsing. Without it, babies develop respiratory distress syndrome, which makes breathing extremely difficult. This is one of the most common complications of prematurity and the reason many premies need breathing support right after birth.
The digestive system is another vulnerability. Premies frequently struggle to feed, and some develop a serious condition called necrotizing enterocolitis, where portions of the intestinal lining become inflamed and damaged. In severe cases this can lead to a hole in the intestine and life-threatening infection.
Other potential complications include jaundice (when the liver isn’t mature enough to clear a waste product called bilirubin from the blood), difficulty regulating body temperature, and heart issues like patent ductus arteriosus, where a blood vessel that should close after birth stays open.
What Happens in the NICU
Most premies spend time in a neonatal intensive care unit, or NICU. The length of stay depends on how early the baby was born. A baby born at 34 weeks might go home in a week or two, while one born at 26 weeks could stay for months.
Incubators are the most recognizable piece of NICU equipment. These clear plastic beds keep the baby warm while allowing nurses and parents to reach in through small ports on the sides. Babies who need breathing help may be placed on a CPAP machine, which pushes a gentle stream of air through a tiny mask or nasal tubes to keep their lung passages open. Babies whose livers are too immature to process bilirubin are treated under bright bili lights, sometimes wrapped in a special light-emitting blanket. Premies who can’t yet suck, swallow, and breathe in a coordinated way are fed through a thin tube threaded through the nose or mouth into the stomach.
One of the simplest and most effective interventions parents can provide is kangaroo care, which is sustained skin-to-skin contact between parent and baby. Premies held skin-to-skin maintain a healthier body temperature, have more stable heart rates, experience fewer episodes of dangerously slow heart rate and drops in oxygen levels, and sleep better. Kangaroo care also supports breastfeeding and has been linked to improved brain development.
Survival by Gestational Age
Survival rates for premies have improved dramatically over the past few decades, especially for the earliest births. Babies born at 28 weeks or later now survive at very high rates in countries with access to modern neonatal care. The picture gets more uncertain the earlier a baby is born. At 22 weeks, which is near the edge of viability, recent data from specialized centers show survival rates around 31%. At 23 weeks and slightly beyond, some centers report survival above 65%, though outcomes vary considerably depending on the hospital’s resources and experience.
Late preterm babies, born between 34 and 37 weeks, survive at rates comparable to full-term infants, though they still face a higher chance of short-term complications like feeding difficulties and jaundice.
Long-Term Development
Many premies grow up perfectly healthy, especially those born after 32 weeks. But prematurity can leave lasting marks. The CDC lists breathing problems, feeding difficulties, cerebral palsy, developmental delays, vision problems, and hearing problems among the potential long-term effects. Risk increases the earlier the baby was born.
One important concept for parents of premies is corrected age (sometimes called adjusted age). Because a premie’s brain and body haven’t developed to the same degree as a full-term infant’s, doctors track milestones using a corrected age rather than the baby’s actual birthday. The formula is simple: subtract the number of weeks premature from the baby’s current age. So a four-month-old who was born eight weeks early has a corrected age of two months, and would be expected to hit milestones on that two-month timeline, not the four-month one.
This adjustment matters because comparing a premie to same-age peers born at full term can make normal development look like a delay. Most pediatricians use corrected age for developmental assessments through at least the first two years of life, and sometimes longer for very early premies.