Pediatrics
Corrected Age for Premature Babies: How to Calculate It, What It's For and When to Stop
A premature baby has two ages: the one on the calendar and the one that matches their development. How the American Academy of Pediatrics calculates corrected age, how it differs from postmenstrual age, and why vaccines are never corrected.
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Corrected Age for Preemies
When a baby arrives early, the family leaves the hospital with two dates that matter: the day the baby was born and the day they were due. The American Academy of Pediatrics (AAP) asks parents to keep both in mind when judging development, because a baby who came eight weeks early has not had the same weeks of maturing as a term baby born on the same day. The gap between those two dates is what turns calendar age into corrected age.
What corrected age is
Corrected age is the age your baby would be if you counted from the due date instead of the birth date. It is also called adjusted age — same idea, different name. The AAP defined it in 2004, together with the other ages of the perinatal period, precisely so that pediatricians, neonatologists and researchers would stop using different words for the same thing.
The calculation is a subtraction. Take your baby's actual age in weeks (the weeks since birth) and subtract the number of weeks they were born before 40. The AAP's own example: a baby born at 32 weeks arrived 8 weeks early. At 4 months old (16 weeks), their corrected age is 2 months, and it is realistic to expect the skills of a 2-month-old born at term, not a 4-month-old.
The four ages of a premature baby
Gestational age
At birth
Completed weeks and days of pregnancy on the day of delivery, for example 32+4.
Chronological age
Since birth
The actual calendar age. It is the one on official documents and the one vaccines follow.
Postmenstrual age
Gestational + chronological
Used mainly in the neonatal unit, while the baby has not yet reached the due date.
Corrected age
Chronological − weeks early
Counts from the due date. It is the one used for development and growth.
How to calculate it, step by step
- Write down the gestational age at birth in weeks and days (it is on the discharge summary).
- Subtract it from 40 weeks: the result is how many weeks early your baby was. A baby born at 33+4 came 6 weeks and 3 days early.
- Count the weeks from birth to today: that is the chronological age.
- Subtract the weeks early from the chronological age. What is left is the corrected age.
- If the result is negative, your baby has not reached the due date yet: during that stretch clinicians talk about postmenstrual age, not corrected age.
Why subtract from 40 if full term is now 39 weeks?
The AAP notes that a pregnancy is now considered full term at 39 weeks, but corrected age is still calculated against 40, which is the reference for the due date. It is not a contradiction: full term describes when it is safe to be born, while 40 weeks sets the point from which the count starts.
What corrected age is used for (and what it isn't)
Its main use is judging development. According to the AAP, during the first 2 years corrected age gives a much better idea of when your baby should reach the usual milestones: holding up their head, smiling, rolling over. Using calendar age leads to needless worry, because it expects skills that belong to weeks the baby has not lived outside the womb. The same goes for weight and length: plotted against WHO charts by chronological age, a preemie looks artificially small.
Vaccines follow the actual age
Corrected age does not apply to the vaccine schedule. The AAP Committee on Infectious Diseases states that medically stable preterm infants should receive full doses of the routine vaccines at the same chronological age as babies born at term. Delaying them "to correct" leaves a baby unprotected exactly when they are most at risk.
Until when to correct
The AAP refers to the first 2 years of life: that is the period in which corrected age helps you know when to expect each milestone. As a child grows, a few weeks' difference matters less and less against their total age. If your baby was born very early, your pediatrician or neonatal follow-up clinic will tell you how long it makes sense to keep correcting in their particular case.
WHO categories of preterm birth
Gestational age at birth
Extremely preterm
< 28 weeks
More than 12 weeks to correct for.
Very preterm
28 to < 32 weeks
Between 8 and 12 weeks to correct for.
Moderate to late preterm
32 to < 37 weeks
Up to 8 weeks to correct for. From 37 weeks there is nothing to correct.
Our corrected age calculator does the arithmetic in exact days: from the birth date and the weeks and days of gestation it gives you the corrected age in months and weeks, the due date, the postmenstrual age and the WHO category. And if you want to compare weight, length or head circumference with the WHO charts, the 0–24 month growth calculator applies corrected age automatically when you enter the weeks of gestation.
Sources
- American Academy of Pediatrics (HealthyChildren.org). Corrected Age For Preemies.
- Engle WA; AAP Committee on Fetus and Newborn (2004). Age terminology during the perinatal period. Pediatrics 114(5):1362-1364.
- Saari TN; AAP Committee on Infectious Diseases (2003). Immunization of preterm and low birth weight infants. Pediatrics 112(1):193-198.
- World Health Organization. Preterm birth (fact sheet).
