If your baby was born early, he really has 2 birthdays. The day your baby was born is the official date of birth, but your original due date is also an important milestone for your baby.
When you measure your baby’s development—that is, when you look at what is appropriate behavior for an infant of your baby’s age—consider both of those dates. You’ll need to determine your baby’s corrected age, or postmenstrual age, to know where he should be developmentally.
Calculating corrected age isn’t difficult. 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. A term pregnancy is 40 weeks’ gestation. To determine the number of weeks premature your baby was at birth, subtract his gestational age at birth from 40. For example, if your son was born at 28 weeks’ gestation, he was 12 weeks (3 months) premature. If he is now 6 months old (24 weeks since birth), his corrected age is 24 weeks - 12 weeks = 12 weeks (3 months).
Actual age in weeks minus Weeks premature = Corrected age
In this case, even if your son is 6 months old, you should expect him to be at or near the developmental level of a 3-month-old term baby. It would be unrealistic to expect your son to be physically ready to sit up with very little support or on his own—a skill that is frequently emerging in term babies around the age of 6 months. Your baby may just be beginning to roll over, which is developmentally normal for a term baby of 3 months and, therefore, for a baby whose age is 3 months corrected.
Parents are often frustrated by well-meaning family and friends who express concerns about their baby’s development. People may think your son is delayed for a 6-month old, for example, when in fact he is performing ahead of his corrected age of 3 months.
Explaining corrected age and why your baby is so small can get tiresome after a while, and many people continue to be confused. Don’t worry—you’ll only need to correct for prematurity until your child reaches the age of 2 to 2 1/2 years. Most will catch up developmentally by this age.