A birth is considered premature if it happens before 37 completed weeks of pregnancy. A full-term pregnancy lasts about 40 weeks, so any baby arriving more than three weeks early falls into the preterm category. Most premature births happen in the later preterm window, between 32 and 36 weeks, but the earlier a baby arrives, the greater the health risks.
Categories of Premature Birth
The World Health Organization breaks preterm birth into three categories based on how early the baby is born:
- Extremely preterm: before 28 weeks
- Very preterm: 28 to less than 32 weeks
- Moderate to late preterm: 32 to 37 weeks
These categories matter because they closely predict the type and severity of complications a baby may face. A baby born at 35 weeks has a very different outlook than one born at 25 weeks, even though both are technically premature.
Why Premature Birth Happens
In many cases, there’s no single clear cause. But several factors raise the risk. Medical conditions during pregnancy play a major role: high blood pressure, diabetes, gestational diabetes, vaginal or urinary tract infections, and abnormal bleeding can all trigger early labor. A short or prematurely thinning cervix is one of the strongest physical predictors, particularly when it shortens during the second trimester rather than the third.
Pregnancy-specific factors also contribute. Pregnancies from in vitro fertilization, pregnancies spaced less than six months apart, and a condition called placenta previa (where the placenta covers the cervical opening) all increase risk. Women who have had a previous cesarean delivery or uterine surgery face a higher chance of uterine complications that can lead to early delivery.
Lifestyle and environmental factors round out the picture. Smoking, alcohol use, drug use, high stress levels, long working hours with extended standing, lack of prenatal care, and exposure to environmental pollutants all elevate the likelihood of preterm labor. Domestic violence, whether physical, sexual, or emotional, is also a recognized risk factor.
Health Risks in the First Weeks
The most common immediate problem for premature babies is difficulty breathing. Their lungs haven’t finished developing, so a condition called respiratory distress syndrome is frequent, especially in earlier arrivals. Among late preterm babies born at 34 weeks, about 11.6% develop respiratory distress. That drops to 5.6% at 35 weeks and 2.3% at 36 weeks. Even at these relatively late stages, the risk is substantially higher than for babies born at full term.
Breathing problems can cascade into other complications. Premature babies with respiratory distress have higher rates of bleeding in the brain, a serious eye condition that can affect vision, bloodstream infections, and a dangerous intestinal condition where portions of the bowel become inflamed or die. At 35 and 36 weeks, babies with respiratory distress face roughly four times the risk of that intestinal complication compared to premature babies breathing normally.
Overall, about half of babies born at 34 weeks experience some form of medical complication. By 36 weeks, that figure drops to about 12%, still meaningfully higher than full-term infants. Late preterm babies are also readmitted to the hospital at more than twice the rate of full-term newborns during the first month of life (3.5% versus 1.5%).
Long-Term Developmental Effects
The earlier a baby is born, the more likely they are to face lasting developmental challenges. These span motor skills, cognition, vision, hearing, and behavior.
Motor development is one of the most visible areas affected. Delays in sitting, walking, and coordination are common among very and extremely preterm children. Cerebral palsy, a group of disorders affecting movement and muscle tone, occurs in roughly 10% to 20% of babies born before 28 weeks. For babies born between 32 and 34 weeks, the rate is closer to 2.4%.
Cognitive effects are well documented. Children born extremely preterm score an average of 11 to 13 points lower on IQ tests compared to children born at full term. Challenges with working memory, self-regulation, and problem-solving are common and often show up as difficulties in math, reading, and spelling that can persist throughout school. A large Norwegian study following over 900,000 people found that those born extremely preterm had significantly higher rates of intellectual disability and were more likely to rely on disability support in adulthood.
Vision problems occur frequently in preterm children, including nearsightedness, misaligned eyes, and reduced visual sharpness. These can develop even in babies who never had the retinal condition most closely associated with prematurity. Hearing impairment, epilepsy, attention-deficit/hyperactivity disorder, and autism spectrum disorder also occur at elevated rates.
What Survival Looks Like at the Earliest Weeks
Viability, the point at which a baby can potentially survive outside the womb, generally begins around 22 to 23 weeks with intensive medical support. Survival at 22 weeks is rare, roughly 3%. By 24 weeks, about 46% of babies survive. At 25 weeks, survival reaches approximately 79%, and by 28 weeks, it climbs above 90%. These numbers reflect outcomes from centers equipped with advanced neonatal intensive care. Survival at the earliest gestational ages varies significantly by hospital resources and location.
How Doctors Reduce the Risks
When preterm delivery appears likely within the next seven days, doctors can give the mother a course of steroid injections. These accelerate the baby’s lung development and dramatically improve outcomes. Babies whose mothers received these steroids have about a third lower risk of respiratory distress, roughly half the risk of brain bleeding, and a 31% lower risk of death compared to babies whose mothers did not receive them. This treatment is recommended between 24 and 36 weeks of gestation.
For women at risk of recurrent preterm birth, two preventive approaches have strong evidence. Vaginal progesterone, a hormone supplement used daily from around 18 to 36 weeks, reduces the chance of early delivery by about 32% in women who have both a history of preterm birth and a shortened cervix (25 millimeters or less on ultrasound). A cervical cerclage, a stitch placed around the cervix to keep it closed, works through a different mechanism but achieves similar results in the same group of women. Notably, progesterone alone does not appear to help women whose only risk factor is a previous preterm birth but whose cervix measures normally. The combination of history plus a short cervix is what identifies women who genuinely benefit from these interventions.
Late Preterm Babies Need Attention Too
Because babies born at 34 to 36 weeks often look and act close to full term, their risks are sometimes underestimated. But the numbers tell a different story. Compared to babies born at 39 to 40 weeks, a baby born at 34 weeks is 40 times more likely to develop respiratory distress. At 35 weeks, the risk is still 22 times higher. Even at 36 weeks, it’s 9 times the rate of a full-term baby.
Beyond breathing, late preterm babies face higher rates of low blood sugar, difficulty maintaining body temperature, jaundice, feeding challenges, and infections. These issues often mean longer initial hospital stays and a greater chance of being readmitted after going home. Parents of late preterm babies benefit from knowing that “almost full term” still carries real, measurable differences in how smoothly the first weeks go.