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How Do You Count A Preemie's Age? Their Due Date & Birthday Can Get Confusing

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There are a million questions that arise when your baby arrives before their due date — it's a really stressful time for families of preemies. My niece was born several weeks before her due date, and though she's now a ridiculously smart and healthy college freshman, it wasn't always clear that would be the case. Especially because she was so tiny, she could fit in one hand. It's hard to let go of that due date in your mind, too, which is why some parents give their baby's "corrected age" But how do you count a preemie's age? Do you note their due date or their birthday?

Your baby's birthday will always be their birthday. However, for their developmental milestones, you do have to calculate their age a bit differently. It's going to require some math. If your baby was born on Feb. 20, but was due April 1, your baby was about five weeks early. That means that they'll hit their major milestones about five weeks behind the allotted schedule. For something like smiling, that may mean it can take your baby until 10 or 11 weeks to smile. For sitting up, they might be about 7 months old, according to the American Academy of Pediatrics (AAP).

My niece "caught up" pretty early on in her toddlerhood, but still had lingering physiological problems that may or may not be related to her premature birth — though they're not severe and in no way hampered her ability to become a national debate champion or champion pain in her uncle's (my husband's) butt. She was a very small infant for the first several months, and, according to Emory University School of Medicine, that's not uncommon. It's also completely individual to the preemie.

While some babies "catch up" within the first few months of birth when considering factors like head circumference and length, most don't fully meet their peers until toddlerhood. However, Emory reminded parents that just because a baby is a preemie doesn't mean they're always going to be small. I am living proof of that. I myself was an early baby, and now I am nearly 6-feet and 155 pounds. I was the tallest kid in all of my classes through junior year of high school.

Do you count a preemie's age by their due date forever? No. Just until they're caught up and developing normally for their age and not their due date. The math is pretty simple, according to the AAP. "Begin with your baby’s actual age in weeks (number of weeks since the date of birth) and then subtract the number of weeks your baby was preterm. This is your baby’s corrected age," the website noted.

As for those milestones, the AAP has guidelines for those as well. They found "the most important thing is to make sure your child is moving forward in his development." It's not going to be the same for every term baby, so it's certainly not going to be the same for every preemie. What you're looking for is momentum — forward progression towards the average range. Preemies may be slower, as there are complexities involved with each case, or they might experience explosive growth as I did and my niece did, even if we both required some interventions like speech (her) and behavioral modifications (me — I was a very hyperactive child).

Your baby is as individual as any other, but with a little extra math to consider.

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