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Adjusted Age Calculator

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Bogna Szyk and Jack Bowater
Based on research by
Villar J, Giuliani F, Barros F, Roggero P, Coronado Zarco IA, Rego MAS, Ochieng R, Gianni ML, Rao S, Lambert A, Ryumina I, Britto C, Chawla D, Cheikh Ismail L, Ali SR, Hirst J, Teji JS, Abawi K, Asibey J, Agyeman-Duah J, McCormick K, Bertino E, Papageorghiou AT, Figueras-Aloy J, Bhutta Z, Kennedy S. Monitoring the Postnatal Growth of Preterm Infants: A Paradigm Change.; Pediatrics; February 2018
Last updated: Jan 18, 2024

Our adjusted age calculator allows you to calculate the corrected age of your newborn baby quickly and easily. This tool is designed for babies born preterm, that is between the 21st and the 37th week of gestation.

Read on to discover why it is so important to know the adjusted age for prematurity and find out how to correctly use our premature age calculator.

🍼 We'll also talk about details and differences in care for babies born before the 37th week of their gestation.

Why do we need the adjusted age calculator for prematurity?

We need to know the adjusted age of a baby to correctly assess its development and milestones. If we didn't use the corrected gestational age calculator, we could draw the false conclusion that the baby is not developing properly, which may lead to the application of unnecessary or even harmful treatments. You should always inform your pediatrician that your baby was born preterm.

After your baby's adjusted age is 24-months, you should start using their chronological age.

📚 Age adjustment for prematurity is a method recommended by:

  • American Academy of Pediatrics (AAP);
  • Centre for Disease Control and Prevention (CDC); and
  • American Congress of Obstetricians and Gynecologists (ACOG).

How to use the adjusted age calculator?

Our tool requires just three easy steps:

  1. Enter your baby's actual birth date.

  2. Choose the method you'd like to use:

    • Gestational age - enter your baby's gestational age at the moment of their birth; or
    • Due date - use the date you were expected to go into labor (this is the same as your expected delivery date. To calculate the value, you can use the due date calculator.
  3. Your results are here!

    • You will receive the value of both your baby's chronological and adjusted age for prematurity; and
    • If you've just welcomed your baby into the world, the calculator will tell you the exact moment you should start counting its adjusted age.

Premature birth is often associated with a medical intervention - check your probability of vaginal delivery after a C-section using the VBAC Calculator.

How to calculate the preemie adjusted age?

Calculating the adjusted age is not as difficult as it seems - try it out!

  1. Find out how many days your baby was born early. This can be done in two ways:
    • Find the difference between the baby's due date and their actual birth date; or
    • Calculate the difference between their gestational age at birth and the full-term gestational age (280 days).
  2. Subtract the number of days early from your baby's current age in days.

💡 Human pregnancy takes 40 weeks, that is 280 days. If the baby was born few days before or after that date, we say that the pregnancy was full term.

The corrected age calculator uses the following equations to achieve the above method:

Chronological age = (Current date - Birth date)

  1. Due date method:

    Age difference = Due date - Birth date

  2. Gestational age method:

    Age difference = Full term (280 days) - Gestational age at the time of birth (in days)

Adjusted age = Chronological age - Age difference

A word on premature babies

The preterm baby is a child that was born before the 37th week of the pregnancy. A baby can be extremely (<28 wks), very (<32 wks), or moderately preterm (<37 wks). Before the delivery, the future mother should be given corticosteroids to speed up the baby's lung development - babies born very preterm may have problems taking their first breath.

💡 The youngest premature baby was born after only 21 weeks and 4 days of the pregnancy and weighed 0.9 lb (410 g) - that's less than the weight of the official NFL football (0.93 lb, 430 g)!

Every newborn baby must be assessed using the Apgar score. The higher the score, the better the result. Preterm babies tend to receive lower scores since their bodies are not fully adapted for the outside world - they usually need full medical attention. Luckily, we created APGAR score calculator as well!

Due to extremely small size of preterm babies and how this impacts their head circumferences, medical staff needs unique methods and equipment to take care of their tiny patients. For example, blood samples are usually taken from the baby's heel.

Due to the structures called fontanelles a physician can take a look into the baby's head using an ultrasound - it's a safe, non-invasive and painless test.

Preterm-born babies have prolonged hyperbilirubinemia of newborns, and are at greater risk of complications due to it. Want to know more about the concept? Use the Omni newborn hyperbilirubinemia assessment calculator.


What is the adjusted age?

Adjusted age is a method of calculating the actual age of babies born before the 37th week of gestation, adjusting for the additional time that would typically be spent in the womb. The adjusted age allows the physicians to properly assess the development of a child, taking into account their preterm delivery date.

We should start using the birth age when the baby's adjusted age reaches 24 months. 👶

Why does my baby's adjusted age differ from the actual age?

If your baby was born preterm, the physicians will usually refer to their adjusted age rather than their chronological (actual) age.

E.g., If your baby was born on the 36th week of the pregnancy, the adjusted age will differ from the actual age by 4 weeks (this is because the typical human pregnancy lasts 40 weeks).

Is the adjusted age the same as corrected age?

Yes, both adjusted age and corrected age describe the same thing - adjusting the age of a preterm baby by considering the difference between the actual birth date and the expected due date.

How to calculate adjusted age for prematurity?

We can calculate adjusted age for a preemie using a few simple equations:

  1. Find the baby's chronological age in days by subtracting their birth date from today's date.
  2. Subtract the baby's gestational age in days at the time of birth from 280 (the full term days).
  3. Subtract the result of Step 2 from the chronological age to get the adjusted age.
Łucja Zaborowska, MD, PhD candidate
Current date
Birth date
Gestational age
Gestational age
Gestational age
Chronological age
Adjusted age
Adj. 18 months
Adj. 22 months
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