Maternal and Child Health Bureau ~ 10
4. Using corrected age to plot measures for low birthweight and premature infants
Low birthweight infants form a heterogeneous group that includes premature infants and those born at term. Therefore, it is important to classify low birth weight infants according to their weight for their gestational age.
Gestational ages of the premature infant may range from 23-38 weeks. Their weights may be classified as appropriate for gestational age (AGA), or small for gestational age (SGA) due to intrauterine growth retardation (IUGR).
Growth charts based on a large longitudinal cohort from the Infant Health and Development Program (IHDP) are available for two birthweights: 1500 grams or less and 1501-2500 grams. (Guo 1997, Guo 1996) When these charts are used for premature infants, the measures should be plotted according to the corrected age. This is also true when the CDC infant (0-36 months) charts are used.
Corrected (or gestation-adjusted) age is determined by subtracting the number of weeks of prematurity from the infant's chronological age. Guidelines for calculating an infant's corrected (or gestation-adjusted) age are provided in Overview of the CDC Growth Charts.
For most premature infants, the corrected age should be used until he/she reaches 2 years when plotting length, weight, and head circumference. For the premature infant weighing less than 1000 grams at birth, the corrected age is often used until age 3 years. If the child's growth "catches up" before 24-36 months of age, chronologic age is used instead of corrected age.
Infants born small for gestational age may fall below the 5th percentile on the CDC charts, even when corrected for gestational age. However, the CDC charts can be used to assess an infant's rate of growth; growth rate should parallel that of the reference infants.