How Many Weeks Is Considered Preterm?

A baby is considered preterm when born before 37 completed weeks of pregnancy.

A pregnant person might think any delivery before the due date counts as “early,” but the medical definition is more precise. Full-term pregnancy typically lasts about 40 weeks, so the line between preterm and full-term matters for care and outcomes.

This article explains the official cutoff, breaks down the four subcategories of preterm birth, and covers what the research says about survival and health at each stage. Knowing where the weeks fall helps you understand what doctors look for and why delivery timing decisions are made.

What Is the Official Definition of Preterm Birth?

Preterm birth is when a baby is born alive before 37 weeks of pregnancy are completed. The World Health Organization and the CDC both use this threshold. A normal pregnancy runs about 40 weeks from the first day of the last menstrual period, so preterm covers everything from around 37 weeks all the way down to the edge of viability.

Within that range, the risks and outcomes vary a great deal. A baby born at 35 weeks has a very different picture than one born at 26 weeks. That’s why doctors divide preterm births into more specific categories based on gestational age.

Why the Week-by-Week Breakdown Matters

It’s natural to hear “preterm” and imagine a standard level of risk. But the week you’re born makes an enormous difference in what kind of care a baby needs and what the outlook looks like. The categories help families and medical teams prepare.

• Late preterm (34–36 weeks): These babies are the most common group, accounting for about 7.4% of all births in the U.S. Many need only a short NICU stay, and some research suggests they have the same chance of being healthy as full-term infants.
• Moderately preterm (32–34 weeks): Babies at this stage often need respiratory support and feeding help, but survival rates are high. A large French study found survival at 32–34 weeks was about 98.9%.
• Very preterm (28–32 weeks): These infants usually require several weeks in the NICU. Survival rates in pooled study data rise to above 95% by the end of this window.
• Extremely preterm (before 28 weeks): This is the highest-risk group. Outcomes depend heavily on the exact week and the resources available. Babies born at 22–24 weeks have a sobering outlook, with only about 1 in 3 surviving.

The categories aren’t just labels—they guide everything from steroid shots for lung development to decisions about whether to attempt resuscitation in the delivery room.

Understanding Survival Rates by Gestational Age

Survival rates for preterm babies have improved dramatically over the last few decades thanks to advances in neonatal intensive care. Still, the numbers shift quickly from week to week, especially before 28 weeks. According to the CDC’s Preterm Birth Definition, any delivery before 37 weeks counts, but the real story is in the subcategories.

At 27 through 31 weeks, a study in JAMA Pediatrics reported survival at about 93.6%. By 32 weeks, that number climbs above 98%. Before 24 weeks, experts at University of Utah Health note that a baby has less than a 50 percent chance at survival. Viability is generally not considered likely until after 24 weeks.

These figures come from large national studies, but individual outcomes depend on the baby’s weight, any infections, and the quality of the NICU. A table can help you see the range at a glance.

Keep in mind that survival is just one piece. Some extremely preterm babies who survive face long-term medical or developmental challenges. That’s why decisions around delivery timing are never taken lightly.

How Doctors Decide When to Deliver Early

Preterm labor sometimes starts on its own, but other times doctors recommend an early delivery because the risks of staying pregnant outweigh the risks of going early. Conditions like preeclampsia, placental problems, or fetal distress can prompt this decision. Delivery timing is carefully considered to balance safety for both mother and baby.

1. Assess viability: Before 24 weeks, the chance of survival is very low, so intensive intervention may not be recommended unless the family and medical team agree on a plan.
2. Use survival data: The numbers for each week range help guide conversations about resuscitation, steroid shots, and transfer to a hospital with a high-level NICU.
3. Consider maternal health: If the mother has a serious condition like severe preeclampsia, delivery may be necessary even at a very early week to prevent life-threatening complications.
4. Watch for late preterm signs: Even after 34 weeks, doctors usually try to let labor come naturally unless there’s a clear medical reason to intervene early.

Every case is individual, and the final decision is made with the family after reviewing the full picture. The categories provide a framework, not a rigid rule.

The Outlook for Late Preterm and Full-Term Babies

Late preterm babies — those born between 34 and 36 weeks — make up the largest slice of preterm births, and their outlook is generally excellent. A fact sheet from NICHD explains that normal pregnancy length is about 40 weeks, but babies who make it to 34 weeks have already completed most of the critical organ development. Many go home within a few days and need only minimal support.

Some research suggests that preemies born after 34 weeks have the same chance of being healthy as full-term infants. That doesn’t mean there’s zero risk — complications like jaundice, feeding difficulties, or temperature instability can still appear — but the vast majority do well long-term.

Doctors still monitor late preterm babies closely for the first few days, but families can usually feel reassured. A simple comparison shows how the outlook changes past 34 weeks.

For families expecting a late preterm baby, the takeaway is hopeful: the hardest hurdles are already behind.

The Bottom Line

Preterm birth starts at 37 weeks, but that one number covers a wide spectrum. The four categories — late, moderate, very, and extremely preterm — give a clearer picture of what each week means for survival and health. Survival rates climb sharply after 28 weeks, and by 34 weeks the outlook is close to that of a full-term baby.

If you’re pregnant and concerned about early labor, your obstetrician or midwife can walk you through the specific risks for your week and any signs to watch for. They’ll also help you understand your hospital’s NICU capabilities and what to expect if delivery comes earlier than planned.