About the Adjusted Age Calculator
This adjusted age calculator works out a premature baby’s corrected age — their age counted from the due date rather than the birth date. Enter the date of birth, how many weeks and days pregnant you were at delivery and the date you want to check, and it shows the chronological age, the adjusted (corrected) age, how early the baby was born and, if the due date has not been reached yet, the baby’s corrected gestational age.
Paediatricians use corrected age to judge development, plot growth charts and set expectations for milestones such as smiling, sitting and walking, because a baby born 10 weeks early has had 10 fewer weeks to develop. Parents of preemies use it to understand why their baby may reach milestones later than full-term peers, and to plan feeding and sleep by developmental rather than calendar age.
Full term is taken as 40 weeks. Babies born at 37 weeks or later are not usually corrected, and most clinicians stop correcting at about two years of chronological age, when most preterm children have caught up.
How to use the adjusted age calculator
- 1Enter your baby’s date of birth.
- 2Enter the gestational age at birth in weeks and days (from the discharge summary).
- 3Choose the date you want the age for — usually today.
- 4Read the adjusted age and use it for milestones and growth charts.
- 5Use the table to plan checkups by adjusted age.
Formula and method
A full-term pregnancy is 40 weeks (280 days). Subtracting the gestational age at birth from 280 days gives how many days early the baby was born, and adding those days to the birth date gives the original due date. The adjusted age is simply the time elapsed since that due date, while chronological age is the time since birth.
Before the due date is reached, the calculator reports a corrected gestational age instead — gestational age at birth plus days since birth — which is how neonatal units describe preterm infants (for example “36 weeks corrected”). Ages are shown as calendar years, months and days.
- GA
- Gestational age at birth (weeks + days)
- 40 weeks
- Full-term reference point (280 days)
Worked examples
Born at 30 weeks on March 10, 2026
Born at 30 weeks, the baby arrived 10 weeks (70 days) early, so the original due date was May 19, 2026. On September 29 the baby is 6 months 19 days old by the calendar but 4 months 10 days adjusted — 19 weeks since the due date.
Still in the NICU before the due date
A baby born at 28 weeks 3 days is 81 days (11.6 weeks) early. On September 29, 59 days after birth, the corrected gestational age is 28w3d + 59 days = 36 weeks 6 days; the original due date is October 21, 2026.
34-weeker at 1 year
Born at 34 weeks 2 days, the baby was 40 days (5.7 weeks) early. On September 29, 2026 the chronological age is 1 year 28 days, but the adjusted age measured from the October 11, 2025 due date is 11 months 18 days.
Frequently asked questions
What is adjusted age for a premature baby?+
Adjusted or corrected age is a preterm baby’s age counted from their due date rather than their birth date. It reflects developmental maturity, so it is used to assess milestones and to plot growth in the first two years.
Until what age should I use adjusted age?+
Most paediatricians correct for prematurity until about 2 years of chronological age. Some continue to 3 years for babies born very early (before 28 weeks) if they are still catching up.
Should I use adjusted age for vaccines?+
No. Vaccinations follow chronological age — the age since birth — even for premature babies. Adjusted age is used for development, growth charts and often for starting solid foods.
Do babies born at 37 weeks need an adjusted age?+
Generally not. Babies born at 37 weeks or later are considered term, and the difference is small enough that most clinicians use chronological age for them.
What is corrected gestational age?+
Before a preterm baby reaches their due date, age is often described as a gestational age — for example “35 weeks corrected” — calculated as gestational age at birth plus the weeks since birth.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.