Your baby’s age

Premature babies are special in a lot of ways. One way in particular is how their age is calculated. Babies that are born full-term have a birthday that falls on whatever day they’re delivered, and that birthday is used as a basis for how their age is calculated as they grow. That isn’t always exactly the case for babies who are born early.

Chronological vs. adjusted age

There are two ways you can measure a preemie’s age. First there’s chronological age, which is the number of days that have passed since the baby was born. That way is pretty normal, but there’s also the baby’s adjusted age, which is measured by subtracting the number of weeks a baby was preterm from his or her chronological age (in weeks).
Here’s an example of an adjusted age: if a baby is born at 30 weeks, he or she is 10 weeks away from being born full-term at 40 weeks, so 10 weeks premature. If the baby is now 28 weeks old, his or her adjusted age will be 28 weeks minus 10 weeks.
To determine your baby’s adjusted age in months, you can calculate their adjusted age in weeks and then divide that number by four.

Adjusted age is most important in babies’ first few years of life, when developmental milestones are roughly estimated by age, since preemies are more developmentally ready to reach milestones at their adjusted age than at their chronological age. They’re also more likely to look their adjusted age.

Preemies and birthdays 

Birthday rules for preemies can get a little bent. In their early years, preemies are sometimes thought to have two days that represent a birthday. The first is the actual day that the baby was born, and the second is the day the baby was originally due.
Having an adjusted age is mostly handy because it wouldn’t really be fair to expect a premature baby to develop at the same rate as a baby that was born full-term on the same day. Most premature babies have some catching up to do, and they usually reach developmental milestones a little later than other babies their age. So as parents watch for these milestones, it’s helpful for them to keep these two birthdays in mind so that you’re looking for developments that are appropriate for a premature baby. That doesn’t mean you preemie can’t have two birthday parties if you want, though!

Talking to other people about your baby’s age

Usually an adjusted age is used with a healthcare provider, while the chronological age is used in more casual settings. But outside of talking with their healthcare provider, there’s no right or wrong way for a parent to tell people their premature baby’s age, and it’s more about what the parent feels comfortable saying.

Usually around the two year-mark, premature babies’ development catches up with that of full-term babies, and parents stop relying on the baby’s adjusted age to notice developmental milestones. But like most things regarding preemies, this depends on things like the baby’s development and what the caregivers prefer.

  • “Corrected Age for Preemies.” Healthy Children. American Academy of Pediatrics, Nov 21 2015. Web.
  • “The premature infant: How old is my baby?” March of Dimes. March of Dimes, August 2014. Web.

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