11.3 Growth Assessment in Preterm & Special Populations

Corrected Age for Preterm Infants

Corrected age = chronological age - number of weeks born preterm. A 6-month-old born 8 weeks early has a corrected age of about 4 months and should be plotted - and developmentally screened - as a 4-month-old. Growth parameters (weight, length, head circumference) are corrected until 24 months, the point by which most preterm infants have caught up. Correcting too short a period over-diagnoses growth faltering; never correcting labels a thriving preterm infant as failing to thrive. Some clinicians continue correcting head circumference beyond 24 months in very preterm infants, but 24 months is the standard exam answer for weight and length.

Intrauterine and NICU Charts

While hospitalized, preterm infants are plotted by postmenstrual age (PMA) on intrauterine-based charts rather than postnatal charts. The Fenton 2013 charts cover roughly 22-50 weeks PMA and remain the most widely used in US NICUs. The Olsen charts and the INTERGROWTH-21st postnatal preterm growth standards are alternatives; Olsen data informed some updated NICU curves, and INTERGROWTH-21st describes growth of preterm infants under optimal care. The practical exam sequence: Fenton (or Olsen/INTERGROWTH) in the NICU to about 44-50 weeks PMA, then transition to the WHO growth standards using corrected age after discharge. Target growth velocity for the growing preterm infant is approximately 15-20 g/kg/day for weight (with roughly 0.9-1.1 cm/week length and 0.7-0.9 cm/week head circumference), aiming to mimic intrauterine rates without exceeding them into rapid catch-up adiposity.

Condition-Specific Growth Charts

• Down syndrome (trisomy 21): syndrome-specific charts (updated 2015 charts for 0-20 years) reflect lower height velocity and shorter adult stature. Children with DS run a higher BMI-for-age on standard charts, but BMI may underestimate adiposity in DS because of reduced lean mass and shorter stature - so interpret BMI cautiously and watch the trend.
• Cerebral palsy: condition-specific charts stratified by GMFCS (Gross Motor Function Classification System) level (Strauss, Brooks/Day, and Stevenson work). Key clinical nuance: low weight can be appropriate in severe, non-ambulatory CP (GMFCS IV-V) because of reduced muscle mass and lower metabolic demand - forcing weight to standard percentiles can cause overfeeding and harm. When stature cannot be measured, use the alternative measures from 11.1 (knee height, ulnar length, tibial length with Stevenson's equations).
• Achondroplasia and other skeletal dysplasias: condition-specific charts are essential because standard charts misclassify every child as profoundly short with an elevated BMI-for-length relationship; syndrome-specific weight-for-height references exist for achondroplasia.
• Turner syndrome and Noonan syndrome: dedicated charts exist for both; Turner charts help identify deviation from an already-low genetic channel that might signal a superimposed problem (for example, celiac disease).

Low Birth Weight and IUGR Follow-Up

Infants born small for gestational age (SGA, typically birth weight below the 10th percentile for gestational age) or with intrauterine growth restriction (IUGR) need closer postnatal monitoring than appropriate-for-gestational-age peers. Most SGA infants show catch-up growth in the first 6-12 months, and about 85-90% catch up by age 2; those who have not caught up by 2-4 years are less likely to do so and may warrant endocrine referral (growth hormone is an approved indication for SGA without catch-up). Catch-up that is too rapid carries its own risk - accelerated early weight gain in SGA infants is associated with later insulin resistance and cardiometabolic disease - so aim for steady, proportional catch-up of weight and length together rather than weight alone. Track these infants on standard charts using corrected age if they were also preterm, and record birth weight, length, and gestational age in the nutrition note so the growth trajectory remains interpretable years later.

Bone Age

Bone age, read from a left hand-wrist radiograph against standards such as Greulich and Pyle, estimates skeletal maturity. A delayed bone age in a short child with a family history of late puberty supports constitutional delay of growth and puberty - the child has remaining growth potential. An advanced bone age suggests precocious puberty or another cause of early epiphyseal fusion, which shortens adult height despite tall current stature. Bone age that roughly matches chronological age in a short child points toward familial short stature or a pathologic cause with little reserve.

Mid-Parental Target Height

Genetic height potential is estimated as the mid-parental (target) height:

• Boys: (father's height + mother's height + 13 cm) / 2
• Girls: (father's height + mother's height - 13 cm) / 2
• Expected range: about plus or minus 8.5 cm (2 SD) around the target.

Worked example: father 180 cm, mother 165 cm. For a boy: (180 + 165 + 13) / 2 = 179 cm; for a girl: (180 + 165 - 13) / 2 = 166 cm. A child whose height channel falls far below the genetic target range (roughly more than 2 SD) deserves evaluation rather than reassurance.

When Standard Charts Do Not Apply and How to Document

Do not force preterm, syndromic, or non-ambulatory patients onto standard curves. Document: which chart and reference population was used, whether age was corrected and by how much, which measurement or proxy was used (and the equation if a proxy), and the rationale. State interpretation relative to the condition-specific reference - for example, weight-for-age at the 25th percentile on GMFCS V-specific curves - rather than quoting a standard-chart percentile that has no meaning for that child.