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How Old Is This Preterm Baby? Understanding Corrected Age

If you currently work with infants or hope to work with infants in the future, you will certainly meet a family with a baby who was born preterm. When you meet an infant who was born preterm, it is important to calculate their corrected age.

A common example of a question we are asked during our Infant Nutrition Essentials Q&A sessions is:

If I’m seeing baby Mary born at 34 weeks who is now six months old, what age should I consider her to be from a developmental standpoint?

In order to answer this question, it’s important to define a few key terms:

Gestational age: This is the number of weeks and days the fetus has developed since the beginning of the pregnancy. It is measured from the 1st day of the last menstrual period. If pregnancy was achieved using reproductive technology, gestational age is calculated by adding two weeks to the date of conception. You may hear someone say, “I’m 34 weeks pregnant, only 6 weeks until I get to meet my little one”. The fetus has a gestational age of 34 weeks.

Full Term Pregnancy: A pregnancy is considered full term if the baby is born between 38 – 42 weeks gestational age. A baby born prior to 37 weeks gestational age is considered preterm.

Chronological age: Days, weeks, months or years elapsed since birth. “I can’t believe Mary is six months old today, it seems like she was born just yesterday”. Mary’s chronological age is six months.

Corrected age: This is the age of the baby after accounting for how many days, weeks or months they were born premature. Although full term is considered 38 – 42 weeks gestational age, when calculating corrected age, 40 weeks gestation is utilized. The recommendation is to calculate corrected age until 36 months of age. “Mary is six months old today, but she was born at 34 weeks gestation”. Mary is about 4.5 months corrected age.

How To Calculate Corrected Age?

With these terms defined, the next step is to understand how to calculate corrected age. There are many different techniques for doing this and over time, you will determine which method works the best in your own practice.

Strategy #1: Use expected date of delivery (EDD)

Most families recall the expected due date of their baby. If the baby was due June 1st but was born April 15th, using this technique, you would plot the baby’s growth with June 1st as the starting point. Therefore, on October 15th, the baby has a chronological age of six months, but will be plotted at about 4.5 months corrected age by using the EDD of June 1st.

Strategy #2: Calculate how much to correct age

Chronological Age in Weeks – [40 weeks – Gestational Age at Birth]

24 weeks – [40 weeks – 34 weeks]

24 weeks – 6 weeks

= 18 weeks corrected age (~ 4.5 months)

Strategy #3: Use the gestational age calculator on


Looking back at the original question, Mary is six months chronological age but ~ 4.5 months corrected age. This corrected age of 4.5 months is where we would expect Mary to be from a developmental standpoint.

Do you want to learn more about developmental milestones and preterm infants? Not sure where to start? Consider joining the next round of our flagship course Infant Nutrition Essentials where we do a deep dive on these topics.

Follow this link for all the details:

What are we up to right now at Nutrition Masterclass:

👉 Our flagship course Infant Nutrition Essentials is open for enrollment!

INE is our comprehensive course for dietitians who want to improve their confidence and skills in infant nutrition. This 32 CPEUs course includes 8 pre-recorded modules, twice monthly Q&As, case studies to apply your knowledge and handy cheat sheets! The next round starts November 1, 2023, so now’s the time to secure your spot!

You can register for Infant Nutrition Essentials here:

Click Here to Register for Infant Nutrition Essentials


What are our friends and colleagues are up to:

👉 Shawna Melbourne, RD, CEDRD-S, Certified Intuitive Eating Counselor

Her course ED for RDs: 1-Year ONLINE Training is now open for enrollment with a November 1st start date.

  • Find your confidence in eating disorder support and care
  • 1-Year Online Training JUST for dietitians, RD2B and nutrition professionals
  • This comprehensive course provides you with everything you need to assess and address eating disorders and disordered eating
  • Includes 8 pre-recorded modules, pre-approved by CDR for 21 CPEUs
  • Weekly live Q&A sessions, monthly live case study review sessions
  • Access to a private, welcoming and rich Facebook group

To find out more about this course and to register:

Click Here for More Information on ED for RDs


We’d love to hear from you. Have questions about infant nutrition or Infant Nutrition Essentials? Email us, let’s chat!

Charlene and Julie

P.S. If you’d love to know more about infant nutrition, check out our flagship course Infant Nutrition Essentials.

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