How to Calculate Baby's Adjusted Age

Learn how old your baby really is based on their due date and birth date

How to Calculate Baby's Adjusted Age

Here's a counterintuitive rule: don't calculate your baby's age using their birth date.

Very few babies arrive on their due date (only around 5 per cent) and if your child was born at say, 37 weeks, they may not be on the same developmental trajectory as a baby born at 39 or 40 weeks.

To be considered full-term, a baby should be born at 39 weeks. Anything before that is technically premature.

Much of your baby's development is tied to time in utero so babies born early may need time to reach full-term development.

Once you've calculated your baby's adjusted age, follow along in our stages of play guide starting with zero to three months, up until four years.

Milestone differences based on corrected age

This is important to consider when you’re comparing the milestones of a full-term birth to that of a premature baby. Adjusted (or corrected age) is when you consider not only the date your baby was born but the date they were expected to be born at 40 weeks.

During the first two years of your baby’s life, considering your baby’s adjusted age will give you a better idea of where they are in terms of reaching developmental goals and milestones.

The easiest way to calculate your baby’s adjusted age is to take their gestational age at birth (ie. 36 weeks) and subtract that from 40 weeks of gestation. This example would make your baby four weeks premature.


During the first two years of their life, you will be looking at and comparing your baby’s development as if they were four weeks younger than their chronological age (ie. the day they were born).

Using the example above, if your baby was born at 36 weeks gestation and is four weeks premature, at five months old you would be measuring their skills at a four-month level. The adjusted age is used up until the age of two years.

photo of new baby sleeping on moms chest

How to know if your baby is on track

Want to see if your child is on track? Read our developmental age-by-age guides and remember: no two children are alike and many develop at their own pace. 

Worried about your little one? Perhaps they are not meeting some of these skills you have read about. You might be wondering when to seek additional support or guidance.

The best way to know when to seek additional support is to trust your parental instincts. If you suspect that your baby or young child might have some delays in certain areas, then trust those instincts.

photo of new baby

Where to find resources if you have concerns


You’ll want to find someone trained in the particular area of concern and usually someone with expertise with the pediatric population.

This could be a pediatric speech and language therapist for language or feeding concerns, an IBCLC (International Board Certified Lactation Consultant) for breastfeeding or bottle feeding questions, or a physical or occupational therapist for concerns around motor milestones.

Occupational therapists can also help with fine motor, play skills, sensory processing differences, and feeding or picky eating. You may also need an assessment done by a Developmental Pediatrician if you feel like there is something more going on such as ADHD or Autism.

Again, you will want to reach out to the professionals, be specific about your concerns and ask the respective provider if they can help with those specific concerns before you commit to the appointment or assessment.

This article was originally published on Mar 29, 2023

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