Corrected age: why premature babies are counted differently

You'll hear it on the unit within days: "he's ten weeks old, but only two weeks corrected." It sounds like bookkeeping, but it's the age doctors, nurses and health visitors use when they look at how your baby is growing and developing. Here is what's behind it.

Three ages for one baby

With a baby born early, three numbers come up.

• Actual age (also called chronological age): the time since birth. It's the age on the birth announcement and on official forms.
• Gestational age at birth: how long the pregnancy lasted, in weeks and days. A baby born at 27+6 arrived after 27 weeks and 6 days.
• Corrected age (sometimes called adjusted age): actual age minus the number of weeks your baby was early, counting from 40 weeks, the due date.

An example. A baby born at 28+0 weeks is 12 weeks early. At an actual age of 20 weeks, their corrected age is 8 weeks. On the due date itself, corrected age is zero.

Hospitals often use a fourth number as well: postmenstrual age (PMA). That's gestational age at birth plus actual age, so the baby in the example is 34 weeks PMA at six weeks old. Until a baby reaches term, growth charts and many plans on the unit run on this number.

Why correct at all?

A baby born 12 weeks early has spent those 12 weeks outside the womb instead of in it. The body and the brain largely develop on their own clock, and that clock started at conception, not at birth.

Compare that baby with term-born babies by actual age and they'll almost always seem behind. That mostly tells you about the method, not the baby. Corrected age gives a fairer comparison: you're looking at babies who have had the same amount of time to grow.

Where you'll come across corrected age

Growth. Until about 50 weeks PMA, growth is often plotted on the Fenton preterm growth chart, which was made for premature babies and runs by gestational age from 22 weeks. After that, it hands over to the WHO Child Growth Standards, which run by age. For a premature baby, you plot on those using corrected age. Many countries also have their own national charts, and your hospital will use the ones it knows.

Development. Developmental checks compare what a baby can do with what most babies do at a given age. For a premature baby that's done at corrected age. In the Netherlands, where we live, the youth health service (JGZ) uses the Van Wiechen developmental assessment, a list of milestones such as "follows with the eyes" or "rolls from back to tummy". The principle is the same elsewhere: milestones are judged by corrected age until about two years.

Follow-up clinics. Babies born very early or very small are often seen in a neonatal follow-up programme, with checks on growth, development, vision and hearing. Those appointments are usually planned by corrected age as well.

Where you don't correct

• Vaccinations follow actual age. The immune system responds to what a baby meets after birth, so your baby has their jabs at the usual age, even if they're still very young corrected. Your clinic or hospital will tell you which schedule applies.
• Official matters such as parental leave, childcare and school entry are based on the date of birth.
• Feeding usually doesn't follow an age chart at first but the hospital's plan, in ml per kg per day. When to start solids is something to discuss with your health visitor or paediatrician. With premature babies they often look at both ages and at the signs your baby is showing.

How long do you keep correcting?

The usual cut-off is two years. By then, many premature babies have caught up some of the gap, and a difference of a few months matters less next to the normal differences between any two toddlers. For babies born extremely early, some doctors keep the early birth in mind for longer. Your paediatrician or health visitor will tell you what applies to your child.

What it means for you as a parent

Mostly, correcting means letting go of comparisons for a while. A cousin born on the same day, at term, is starting from somewhere else. It helps to ask which age belongs to the question in front of you:

• Is it about growth or development? Use corrected age.
• Is it about vaccinations or paperwork? Use actual age.
• Not sure? Ask the doctor or nurse which age they mean. It's a question they hear all the time.

Sources

• Fenton TR, Kim JH. A systematic review and meta-analysis to revise the Fenton growth chart for preterm infants. BMC Pediatrics, 2013.
• World Health Organization. WHO Child Growth Standards, 2006.
• The Van Wiechen developmental assessment, as used by the Dutch youth health service (JGZ).

About Hidje

Hidje is the app we made for our own son. It always shows actual age and corrected age side by side, plots weight, length and head circumference on the Fenton chart and then the WHO chart, and lists the Van Wiechen milestones by corrected age. It does the arithmetic for you. What the numbers mean for your baby is a conversation for your own doctor.