What Prematurity Means and the Categories

The definition, and the four bands

The World Health Organization defines preterm as "babies born alive before 37 weeks of pregnancy are completed". That single line does a lot of work. It is about gestation, not about weight, and it is about completed weeks, so a baby born at 36 weeks and six days is preterm and a baby born at 37 weeks and one day is not.

Underneath the headline definition the WHO uses three sub-categories, based on gestational age:

• Extremely preterm — less than 28 weeks
• Very preterm — 28 to less than 32 weeks
• Moderate to late preterm — 32 to 37 weeks

You will also hear "late preterm" used on its own for babies born from 34 weeks up to 36 weeks and six days, and "micro preemie" used informally, mostly in the United States, for the very smallest babies. Micro preemie has no agreed definition and no official body publishes one, so it means whatever the person saying it wants it to mean.

What the UK numbers actually say

The NHS puts it simply: "About 8 out of 100 babies will be born prematurely." Bliss, the UK neonatal charity, estimates that nearly 58,000 babies are born prematurely in the UK every year, which works out at 1 in every 13 babies. Bliss also reports that the rate is rising — the proportion of preterm live births went from 7.4% in 2020 to 7.6% in 2021 — and that in 2021, 8.7% of live births in the Black ethnic group were preterm, the highest of any ethnic group since the data collection began in 2007. That page was last updated in April 2024 and draws on the Neonatal Data Analysis Unit.

Globally the WHO estimates 13.4 million babies were born preterm in 2020, more than 1 in 10, with national rates ranging from 4% to 16%.

Preterm is not the same as needing neonatal care

This is the distinction that gets lost. Bliss estimates that for every 1,000 babies who need neonatal care in the UK, 641 were born at full term. Only 25 were extremely preterm, 51 very preterm and 283 moderately preterm. Read that the other way round and two things follow. Most babies on a neonatal unit were not born early at all. And most babies born late preterm, at 34 to 36 weeks, will not need a neonatal unit — many go to the postnatal ward or transitional care with their mother.

So "premature" and "in the neonatal unit" are two different facts about your baby. Knowing which one applies changes what the next few weeks look like far more than the gestational age on its own.

Late preterm is not "nearly term"

Babies born at 34 to 36 weeks often look robust, feed reasonably and go home quickly, and everybody relaxes. They are still more likely than term babies to have trouble keeping warm, to feed slowly, to get jaundiced and to be readmitted in the first fortnight. The NHS is blunt about the underlying reason: "The earlier in the pregnancy a baby is born, the more vulnerable they are." That is a gradient, not a cliff edge at 37 weeks. If your baby was born at 35 weeks and you are being told to expect a term-baby feeding pattern, that expectation is wrong, not your baby.

Why this page does not give survival rates by week

"Premature baby survival rate by week" is one of the most searched phrases in this whole subject, and we are not going to answer it with a number. Outcome figures by gestation change with the year of the cohort, the country, whether the count starts at admission or at birth, and whether babies who were not given intensive care are included. A percentage lifted out of that context is not information, it is a shape you can misread in the worst week of your life.

What can be said, from the WHO, is that survival at a given gestation depends enormously on where a baby is born: more than 90% of extremely preterm babies born in low-income countries die within the first few days, while in high-income settings fewer than 10% do. The NHS states that "it's possible for a baby to survive if born around 24 weeks of pregnancy onwards." For babies born before 27 weeks in the UK, BAPM publishes a framework that grades risk from extremely high to moderate and is used by the maternity and neonatal teams together to counsel families case by case. Your own team has your scan measurements, your baby's condition and your local outcome data. Ask them. They are the only people who can give you a figure that means anything about your baby.

The words you will hear next

A few terms arrive fast and nobody stops to define them:

• Gestational age — how many weeks pregnant you were at birth.
• Corrected age (or adjusted age) — your baby's age counted from your due date rather than the birth date. This is the one used for development.
• Postmenstrual age or PMA — gestational age plus the weeks since birth. The unit will use this constantly: "she is 34 weeks corrected" means 34 weeks PMA.
• Low birthweight — under 2,500g. It overlaps with prematurity but is a separate thing; a growth-restricted term baby can be low birthweight without being preterm.

What tends to happen next

If your baby needs a unit, the level of unit is chosen by how much support your baby needs, and it can change. If your baby does not, you may still be asked to stay a few extra days for feeding and temperature. Either way, the two things that will matter most over the next year are feeding and corrected age — and both are things you can get properly right with the team's help.