6.1 Growth Parameters & Growth Charts

Domain II.A.1 asks the CPNP-PC to evaluate and interpret growth parameters. That is a plotting-and-trajectory skill, not a request to memorize every CDC cell. You choose the correct chart, obtain the correct measurements, interpret channel and velocity, and know when a number is a pattern that needs workup rather than a one-time small child.

Which chart, and why it matters

CDC and the AAP recommend WHO growth standards from birth to 2 years and CDC growth charts from 2 through 19 years. WHO charts describe how children should grow under optimal conditions (the WHO Multicentre Growth Reference Study of predominantly breastfed infants). CDC 2000 charts describe how U.S. children did grow, including a mixed-fed population. A breastfed 6-month-old plotted on a CDC infant chart can look falsely small; a formula-fed infant can look falsely large. Using CDC charts in infants is a high-yield CPNP-PC trap.

Use sex-specific charts only. Plot immediately. Recheck any surprising point before you order laboratories or label the child with failure to thrive.

Measurement technique is part of interpretation:

• Length until 24 months: two people, head against the board, legs fully extended, dry diaper. Standing a 20-month-old against a wall underestimates length and can fake a drop.
• At 24 months, children often appear to fall a percentile when you switch from length to height because standing height is typically about 0.5–1 cm less than recumbent length. Confirm technique before you diagnose faltering.
• Weigh infants naked or in a dry diaper on a calibrated infant scale. Older children in light clothing, shoes off, same scale when you are chasing a trend.
• Head circumference: occipital-frontal maximum, tape snug, plot on the same visit as weight and length. Skipping the tape after the newborn nursery misses both craniosynostosis and hydrocephalus.
• Do not use BMI as the primary adiposity index before age 2. Under 2 years, weight-for-length is the index. BMI-for-age starts at 2 years.

Prematurity: correct age until 24 months

Infants born before 37 weeks are plotted by corrected (adjusted) age until 24 months chronological age. Corrected age equals chronological age minus weeks of prematurity (weeks before 40). A child who is 6 months chronological and was born at 32 weeks is plotted at 4 months.

Forgetting to correct prematurity makes a healthy former 28-week infant look like failure to thrive or microcephaly. Continuing to correct after 24 months can hide true short stature. Catch-up is expected but not guaranteed; persistent faltering after correct plotting still needs evaluation. Corrected age is also used for developmental surveillance in early infancy, but this section's hard growth rule is plot to 24 months.

Crossing percentiles is a trajectory problem

A single percentile is a snapshot. Velocity and channel are what you interpret.

Major percentile lines commonly displayed are the 3rd (or 5th), 10th, 25th, 50th, 75th, 90th, and 97th (or 95th). Crossing two or more major percentile lines downward is a classic trigger to recheck measurements, feeding, and illness — and it is one of the commonly taught failure-to-thrive (FTT) criteria. Crossing two major lines upward is equally a signal: excess caloric intake, catch-up after illness, or, for head circumference, hydrocephalus or extra-axial fluid versus familial macrocephaly.

A child who has always tracked the 10th percentile with short parents is not automatically pathologic. A child who lived on the 75th and is now on the 10th with falling weight-for-length is a different patient.

Failure to thrive: principles, not one magic cutoff

No single cutoff is universal. Teach the commonly used operational criteria so you recognize them on items, and remember that FTT is a pattern of inadequate growth, usually weight first:

• Crossing two or more major weight percentile lines downward
• Weight below the 5th percentile for age
• Weight-for-length below the 2nd percentile (WHO) under 2 years, or BMI below the 5th percentile after age 2
• Older literature also used weight less than 80% of median weight-for-length

Confirm the measurement before you diagnose. Recheck the scale, the length board, the sex of the chart, and whether a preterm infant was plotted at corrected age. Then take a feeding, stool, vomiting, social, and illness history. Organic versus nonorganic is an outdated binary; most children have mixed contributors (intake, absorption, increased need, and psychosocial context). Weight typically falls first, then length/height, then head circumference. If head circumference falls first, think neurologic, genetic, or craniosynostosis — not just not eating.

Detailed FTT laboratories and nutrition rehabilitation live in later nutrition chapters. This section's job is to recognize the pattern and not wait for an extreme z-score if velocity has already collapsed.

Short stature: familial, constitutional delay, endocrine

Short stature is typically height below the 3rd percentile or more than 2 SD below the mean. The exam cares that you separate normal variants from pathologic poor velocity.

Familial (genetic) short stature. Parents are short. Growth velocity is normal. Bone age equals chronologic age. The child is tracking toward a mid-parental target that is itself short. Continued plotting is appropriate; growth-hormone referral is not the first move.

Constitutional delay of growth and puberty. Family history of late bloomers. Velocity is near-normal after the early years. Bone age is delayed. Puberty is late; adult height is usually in the family range. The child looks young for age.

Pathologic, endocrine, or chronic disease. Height velocity is poor (falling off the height curve). Bone age is often delayed. Clues include midline defects, micropenis, prolonged jaundice (hypopituitarism), constipation/bradycardia/goiter (hypothyroidism), cushingoid features, disproportionate short limbs or trunk (skeletal dysplasia), or chronic disease (celiac disease, IBD, renal disease, congenital heart disease). Disproportionate short stature is not constitutional delay.

Mid-parental (target) height:

• Boys: (father's height + mother's height + 13 cm) / 2, or add 5 inches
• Girls: (father's height + mother's height − 13 cm) / 2, or subtract 5 inches

A commonly used target range is about ±8.5 cm (roughly 2 SD) around mid-parental height. A child growing far below the parental target, or with falling height velocity, is not just a short family.

Microcephaly and macrocephaly

Microcephaly is head circumference more than 2 SD below the mean (often taught as below the 3rd percentile). Primary (prenatal) microcephaly raises genetic, teratogen, and congenital-infection questions. Secondary (postnatal fall-off of a previously normal HC) raises injury, metabolic, or progressive neurologic disease.

Macrocephaly is HC more than 2 SD above the mean (often above the 97th). Familial macrocephaly with a typically developing child and large-headed parents is common. Red flags: crossing HC percentiles upward, sunsetting eyes, bulging fontanelle, irritability, vomiting, or developmental lag — image for hydrocephalus or extra-axial collections rather than reassuring on family history alone.

Always plot HC on the same infant visit as weight and length. A well-baby weight check that skips the tape measure misses craniosynostosis and hydrocephalus.

BMI and obesity from age 2

Starting at 2 years, plot BMI-for-age on CDC charts.

Obesity is BMI-for-age ≥95th percentile for age and sex. Do not apply an adult cutoff of 30 kg/m² to a school-age child. Do not diagnose obesity from weight-for-age alone; a tall child can be heavy without an elevated BMI.

Clinic vignettes

A former 30-week infant is 12 months chronological. Weight sits at the 5th percentile when plotted at 12 months. Corrected age is about 9.5 months; replotted, weight is near the 25th. Do not launch an FTT laboratory panel until corrected age is on the chart.

A 3-year-old's height has gone from the 50th to the 10th over 18 months. Mid-parental height is at the 50th. This is not familial short stature. Poor velocity plus a mismatch to parental target is an endocrine or chronic-disease evaluation.

A 15-month-old's EHR auto-calculates BMI because the default template says pediatric BMI. Switch to WHO weight-for-length. BMI is not the index under 2 years.

Exam traps

1. CDC charts in infants. WHO from birth to 2 years; CDC thereafter.
2. Forgetting corrected age until 24 months in preterm infants.
3. Standing height before 24 months, or treating the expected length-to-height drop as FTT.
4. BMI before age 2 instead of weight-for-length.
5. Calling a child who has always been on the 10th percentile FTT when parents are short and velocity is normal.
6. Skipping head circumference after the newborn nursery.
7. Applying adult BMI cutoffs or diagnosing obesity from weight-for-age alone.

Plot the right chart, correct prematurity, compare to parents, and interpret trend. That is II.A.1.