If your baby was born early you may be used to hearing that your baby is “premature” or a “preemie”. You may also have heard terms such as post-menstrual age, gestational age, corrected age and chronological age. It may be confusing to figure out exactly how old your baby is developmentally, and why it is important to consider more than just their date of birth when calculating age.
What Do These Terms Mean?
Full-Term Gestation: A typical time from last menstrual period to birth that equals 40 weeks. (Range is 38-42 weeks)
Preterm Gestation: Anytime a baby is born before completing at least 38 weeks in the womb.
Chronological age: Simply the number of days, weeks, months, or years since birth.
Gestational age (GA): The completed number of weeks and days of an infant’s time in the womb from the day of the last menstrual period until the day of delivery. A “full term” gestational period is about 40 weeks. A baby born 10 weeks early has a gestational age of 30 weeks:
40 weeks expected term gestation – 10 weeks = 30 weeks GA
Post-menstrual age (PMA): The baby’s gestational age plus the number of weeks and days since birth (chronological age). For example, the preterm baby who was born at 30 weeks gestational age and is now eight weeks old is considered to be 38 weeks post-menstrual age:
30 weeks GA + 8 weeks = 38 weeks PMA
Corrected age: The age of a premature infant or toddler minus the number of weeks or months the baby was born prematurely. For a baby born at 30 weeks gestational age who is now 16 weeks (four months) old chronologically, the number of weeks the baby was born early are subtracted. Your baby’s age is corrected in this way until two years of age:
16 weeks since birth – 10 weeks early = 8 weeks (2 months) corrected age
What Do All of These Numbers Mean?
If your baby was born early, it is important to pay attention to their development and ensure they are reaching each of their milestones. Being born early means your baby had fewer weeks or months to complete their basic brain and body development compared to a baby who was born full-term.
This means that while your early-born baby is growing and developing on the outside, he still needs the full 40 weeks to be ready to do what other full-term infants can do. For example, if you put a four-month-old baby who was born full term next to a four-month-old baby who was born two months early, you may notice a difference in size and skills. The preterm infant may look to be delayed developmentally compared to the full term infant. If you “correct” the baby’s age to two months (chronological age minus number of weeks and divided by months early, equals corrected age) and put him next to a baby who was full term and is also two months old, you will now see many more similarities.
Your pediatrician and other care team members will use your baby’s corrected age to keep track of all of their new and emerging skills for the first two years. By using the corrected age, your baby gets credit for being born early. This is important so that you and others know what to expect about your baby’s development. A four-month-old baby may be expected to have head control but when the preterm baby’s age is corrected to two months, then not having head control yet is okay. Many babies catch up to their full-term peers by two years and correcting for their prematurity is no longer needed.
You may be wondering how to explain this when people ask, “How old is your baby”? You can simply say how many months old your baby is and use the corrected or the chronological age, whatever you prefer. When should you celebrate your baby’s first birthday? You can celebrate on your baby’s birthdate and maybe again when their corrected age is 12 months if you like, for twice the fun!
Jennifer Hofherr is a Neonatology Clinical Therapies Manager at Nationwide Children's Hospital.
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