To calculate adjusted age, subtract the number of weeks your baby was born early from their actual age since birth. This gives you a more accurate picture of where your premature baby should be developmentally, since they missed out on crucial growing time in the womb. Adjusted age (also called corrected age) is used for the first two years of life.
The Formula
The calculation has two simple steps:
- Step 1: Find how many weeks early your baby was born. Subtract their gestational age at birth from 40 weeks (a full-term pregnancy).
- Step 2: Subtract that number from your baby’s current age since birth.
The result is your baby’s adjusted age.
A Worked Example
Say your baby was born at 28 weeks gestation. That’s 12 weeks early (40 minus 28). Now say it’s been 6 months since the day your baby was actually born. Subtract the 12 weeks of prematurity, which equals 3 months, from the 6 months of actual age. Your baby’s adjusted age is 3 months.
This means that when you’re looking at developmental milestones, you’d compare your baby to a typical 3-month-old, not a 6-month-old. That distinction matters a lot. A baby who seems “behind” by chronological standards may be right on track when you account for their early arrival.
Why Adjusted Age Matters
A baby born at 30 weeks missed 10 weeks of brain growth, weight gain, and organ maturation that would have happened in the womb. Their neurological and physical development doesn’t speed up to compensate for leaving early. It continues at roughly the same pace it would have followed inside the uterus.
Comparing a premature baby to full-term babies of the same chronological age creates a misleading picture. A 6-month-old who was born 3 months early has had the same amount of developmental time as a typical 3-month-old. Using adjusted age for milestone tracking puts premature and full-term babies on a level playing field, so parents and clinicians can spot genuine delays rather than false alarms.
When to Use Adjusted Age (and When Not To)
Adjusted age applies to developmental milestones: rolling over, sitting up, babbling, walking, and similar markers. It’s also used when plotting your baby’s height, weight, and head circumference on growth charts. After your baby reaches term-equivalent age (what would have been their due date), standard growth charts can be used with the corrected age until age 2.
Vaccinations are the big exception. All routine childhood vaccines follow chronological age, the time since your baby’s actual birth date. A premature baby should receive their first round of vaccines at 2 months of chronological age, even if they’re still in the hospital. This applies to every vaccine on the standard schedule.
Starting Solid Foods
Introducing solids sits somewhere in between. Most healthy premature babies are ready for solids between 5 and 7 months of actual age. However, 3 months corrected age (3 months after the original due date) is considered the earliest point to even begin thinking about it. Many babies won’t show signs of readiness until later. Look for the same cues you’d watch for in any baby: good head control, interest in food, and the ability to sit with support.
How Long to Keep Using It
The American Academy of Pediatrics recommends using adjusted age until your child turns 2 years old (chronological age). After 24 months, most premature children have caught up to the typical milestone range and the gap between adjusted and chronological age becomes less meaningful.
If your child hasn’t caught up by age 2, they may benefit from continued developmental support. But the standard practice for milestone tracking and growth chart plotting shifts to chronological age at that point.
Quick Reference by Gestational Age
Here’s how the adjustment breaks down for common preterm birth points, assuming a full-term pregnancy of 40 weeks:
- Born at 36 weeks: Subtract 4 weeks (1 month) from actual age
- Born at 34 weeks: Subtract 6 weeks (1.5 months) from actual age
- Born at 32 weeks: Subtract 8 weeks (2 months) from actual age
- Born at 28 weeks: Subtract 12 weeks (3 months) from actual age
- Born at 24 weeks: Subtract 16 weeks (4 months) from actual age
When converting weeks to months for this calculation, use 4-week months rather than calendar months. So 12 weeks premature equals 3 months, not “almost 3 months.”
How Adjusted Age Affects Milestone Expectations
If your baby was born at 32 weeks and is now 6 months old by chronological age, their adjusted age is 4 months. You’d expect them to be working on 4-month skills like holding their head steady, tracking objects with their eyes, and beginning to reach for toys. Walking, which typically happens around 12 months, wouldn’t be expected until about 14 months chronological age for this baby.
This recalibration applies across the board: motor skills, language, social behavior, and cognitive development. It’s not just physical size. A premature baby’s brain is also younger than their birth date suggests, and that shows up in every domain of development. By age 2, the gap narrows enough that most children no longer need the correction. But during those first two years, adjusted age is the more honest number.