While full-term babies are evaluated based on their actual age, a preemie is assessed based on her corrected age (also called adjusted age), or how old she'd be if she had been born on her due date. Take, for example, a baby born three months early: When she's 6 months old, we don't expect her to do 6-month-old tasks -- we expect her to do 3-month-old tasks. If she's meeting the 3-month milestones, then she's developing appropriately for her corrected age. By age 2, the range of normal development is so wide that there aren't distinctions.
Vaccines are the one exception to the corrected-age rule. Immunizations should be given based on a child's chronological age.
A preterm baby's physical growth is also assessed based on adjusted age. Your child's doctor will be looking at how quickly your baby is growing and will want to see that he is growing parallel to the growth curve, even if his height or weight (or both) is below the average for his age group. Premature babies who were born extremely early, who were extremely small for their gestational age, or who have severe ongoing medical problems, may be small compared to their peers for some years. Many of them will continue to grow somewhat faster than their peers through the elementary school years, catching up to the average, year by year, for quite a while. --Kristen Finello
Originally published in American Baby magazine, November 2006. Updated 2009