Corrected age for preemies on a growth chart

How to work out corrected age

AAP's method is simple. Start with your baby's age in weeks since birth, then subtract the number of weeks your baby was born early. To find the weeks early, subtract the gestational age at birth from 40. ¹

AAP's own example: a baby born at 32 weeks was 8 weeks early. At 16 weeks since birth (about 4 months), her corrected age is 8 weeks, about 2 months. You would expect the skills of a 2-month-old born at term, not of a 4-month-old. ¹

childow works in days, which is more exact: it takes the gestational age at birth in weeks and days (for example 33+4), counts days since birth, and subtracts the difference from 40+0.

Who it applies to, and for how long

childow applies the correction for babies born before 37 weeks, for the first 2 years of age since birth. AAP's guidance refers to the first 2 years: "During the first 2 years, using your baby's corrected age will give you a better idea when he or she should reach common developmental goals." ¹ After that, children are compared by their age since birth. Your NICU or pediatrician may use a different cut-off for a particular child; follow their advice for your baby.

For babies born at 37 weeks or later, no correction is applied. The rule is shown on the chart every time it is used, together with the corrected age.

Why it matters on a growth chart

A baby born 8 weeks early who is 4 months old has had 8 fewer weeks to grow than a 4-month-old born at term. Comparing her with term 4-month-olds will usually put her far down the chart, even when she is growing well for her corrected age.

An example from the WHO tables: a girl born at 32 weeks who weighs 5.2 kg at 4 months since birth is at about the 4th percentile if you use her birth date, and at about the 54th percentile if you use her corrected age of about 2 months. Same baby, same weight; the corrected number is the one that describes her growth.

Many simple online calculators only take a birth date. If your baby was born early and a calculator shows a very low percentile, check whether it allowed for prematurity.

Before 40 weeks: why nothing is plotted

The WHO growth standards describe children born at term. The study that produced them excluded babies born before 37 weeks. ² Their curves start at birth for a baby born at about 40 weeks, so they have nothing to say about a baby who is still, in effect, before their due date.

Neonatal units use preterm growth charts for that period, such as the Fenton charts. Those charts are licensed by their authors and are not ours to reproduce, so we do not plot or imitate them. ³ On childow, a measurement taken before your baby reaches 40 weeks' corrected (postmenstrual) age is saved but not plotted, and the chart says: "Before term, NICUs use preterm charts such as Fenton. Your NICU's chart is the right one for now." Plotting starts automatically once your baby reaches term-corrected age.

Corrected age and milestones

The same logic applies to development. CDC's milestone checklists describe what most children can do by a given age. For a baby born early, compare with the checklist for the corrected age during the first 2 years. ^1,4 childow's milestone scheduler sets checkpoint dates from the due date for babies born before 37 weeks, up to the 24-month checkpoint, and from the birth date after that.

If you have concerns about your preterm baby's development, you do not need to wait for corrected age to "catch up". Talk to your pediatrician or NICU follow-up clinic, and you can contact your state's early intervention program yourself; a doctor's referral is not required. Many babies born very early are followed by a developmental clinic after discharge, and that team is the right first call.

Practical tips

• Keep the due date with the birth date. Both appear on every growth record and checkup report.
• Enter gestational age in weeks and days from the discharge papers, for example 31+5.
• Ask which age the clinic is using. If your pediatrician's chart and yours differ, corrected age is one of the first things to check, along with which chart (WHO or CDC 2000) was used.
• Expect the correction to fade in importance. At 3 months, 8 weeks early is a large share of your baby's life. At 20 months it is a much smaller share, which is part of why the correction stops at 2.