What Is Adjusted Age, and Why Does It Matter?

If your baby was born early, you’ve probably had this confusing moment: someone asks how old your baby is, and you genuinely don’t know how to answer. Do you count from the day they were born? From their original due date? And when the pediatrician plots them on a growth chart and frowns—are they really “behind,” or are they being measured against the wrong yardstick?

This is the single most important idea for preemie parents to understand, and it’s the reason this whole site is named Adjusted Age. My son was born at 29 weeks—11 weeks early—and spent 45 days in the NICU. Once we got home, no one clearly explained why the milestone charts felt so wrong, or how one simple calculation would make them make sense. This is the explanation I wish I’d had.

What Adjusted Age Actually Means

Adjusted age is your baby’s age counted from their original due date instead of their birth date. You’ll also hear it called corrected age. The two terms mean the same thing, and you’ll see both on this site and in your baby’s paperwork.

The idea is simple and kind. A baby born 11 weeks early missed 11 weeks of growing in the womb, so when we look at development, we give those weeks back on paper. The American Academy of Pediatrics (AAP) puts it plainly: during the first 2 years, your baby’s corrected age gives you a better idea of when to expect common developmental skills.

How to Calculate Adjusted Age

You need two numbers: your baby’s gestational age at birth (how many weeks pregnant you were when they were born) and how old they are today.

Step 1 — Find how many weeks early your baby was born. Subtract their gestational age at birth from 40.

Step 2 — Subtract those weeks from your baby’s age since birth. What’s left is their adjusted age.

Here’s how that works for three different babies:

• Born at 29 weeks, now 26 weeks old (about 6 months): 40 − 29 = 11 weeks early. 26 − 11 = 15 weeks adjusted, or about 3½ months. That’s my son’s math at 6 months.
• Born at 34 weeks, now 16 weeks old (about 4 months): 40 − 34 = 6 weeks early. 16 − 6 = 10 weeks adjusted, or just over 2 months.
• Born at 36 weeks, now 12 weeks old (about 3 months): 40 − 36 = 4 weeks early. 12 − 4 = 8 weeks adjusted, or almost 2 months.

So when that first baby is doing what a 3½-month-old does, they’re right on track—even though the calendar says 6 months. That gap is normal and expected.

Two quick notes. If you know your due date, you can skip Step 1: the number of weeks between your baby’s birth date and due date is how early they were. And although a pregnancy is now considered full term at 39 weeks, the AAP’s formula still subtracts from 40, so 40 is the number to use.

If doing this in your head at 2am sounds like too much, that’s exactly what our Adjusted Age Calculator is for. Enter your due date or gestational age at birth and it does both steps for you.

Adjusted Age vs. Chronological Age: Which One When?

Chronological age is your baby’s age counted from their birth date—the age on the birth certificate. Both ages matter, just for different things:

• Milestones and development: adjusted age. This is what the AAP means when it says to use corrected age during the first 2 years.
• Growth: your pediatrician may plot your baby’s weight, length and head size using corrected age. Ask which age they’re using (more on growth charts below).
• Vaccines: chronological age. The CDC says preterm babies, whatever their birth weight, get their vaccines at the same chronological age and on the same schedule as full-term babies. The main exception is the timing of the first hepatitis B dose for babies who weighed under 2,000 grams (about 4 lbs 6 oz) at birth, and your NICU team handles that one.
• School: chronological age. Districts go by the birth date on the birth certificate—we cover what that means in Adjusted Age and School Cutoffs.
• Birthdays: chronological age. Celebrate the day they arrived.

What about gestational age and PMA?

In the NICU you’ll hear two more terms. Gestational age is how far along the pregnancy was when your baby was born—it’s the “29 weeks” in “born at 29 weeks.” Postmenstrual age (PMA) is gestational age plus the time since birth, so a baby born at 29 weeks who is now 3 weeks old is 32 weeks PMA. NICU teams usually talk in PMA during the hospital stay; once you’re home, adjusted age is the number you’ll use most. Our NICU Day Counter works out day of life and PMA for you.

Growth Charts and Adjusted Age

Growth charts are where a lot of preemie parents get their first scare, so it helps to know which chart your baby is on.

In the NICU, many teams track growth on a preterm growth chart. A widely used one is the Fenton Preterm Growth Chart, which is designed for babies born before 37 weeks and was revised to line up with the World Health Organization (WHO) growth standards used after it.

For babies from birth to 2 years, the CDC points US doctors to the WHO growth standards. For a preemie, your pediatrician may plot your baby’s growth using corrected age. Practices differ, so ask which age and which chart they’re using—it makes the percentile much easier to understand, and it’s a completely normal question to ask.

Milestones and the 2022 CDC Update

Milestones are where you’ll use adjusted age most often. When you check a milestone list, use the one for your baby’s adjusted age, not their age since birth. A 6-month-old born 11 weeks early should be compared with the list for about 3½ months.

In 2022, the CDC and AAP updated their milestone checklists. Each milestone is now a skill that most children—75% or more—can do by that age. Some milestones moved to different ages as part of that update, so a checklist printed before 2022 may not match the current one.

Our month-by-month guide to milestones by corrected age walks through each checkpoint, and the Milestone Tracker does the adjusting for you and gives you a summary to print for your pediatrician.

How Long Do You Use Adjusted Age?

Most pediatricians use adjusted age until your child is 2 years old by the calendar (24 months chronological age). By then, many preemies have caught up, and the gap between the two ages matters less and less.

For babies born very early, your care team may keep adjusted age in mind for longer. There’s no single switch-off date; it’s a call your pediatrician makes for your child. We go deeper in When to Stop Adjusting Age for Preemies.

What to Bring Up at the Pediatrician

Bring two dates to every visit: your baby’s birth date and their original due date. These questions are worth asking:

• Are you checking milestones against my baby’s adjusted age or chronological age?
• Which age and which chart are you using to plot growth?
• Which milestones should we watch for at this adjusted age?
• Is anything you’re seeing worth an early intervention evaluation?
• When do you plan to stop adjusting for my baby?
• Is my baby up to date on vaccines for their chronological age?

If you’re worried about your baby’s development, you don’t have to wait for a referral. The CDC says a doctor’s referral isn’t necessary for early intervention: if your child is under 3, you can call your state’s early intervention program yourself and ask for an evaluation. These programs are free or reduced cost for children who qualify.

A Word of Reassurance

When you start using adjusted age, a lot of anxiety quietly lifts. I spent more nights than I needed to worrying about whether my son was “behind,” when he was right on track for a baby who came 11 weeks early. The point of adjusted age isn’t to make excuses—it’s to measure your child fairly, against the timeline their body is actually on.

That’s our story. Every preemie follows their own path, and your pediatrician is the right person to walk through your baby’s with you.