Growth Percentile & Z-Score Glossary for Parents

July 15, 2026

Growth chart reports use a handful of terms that rarely get defined in the pediatrician’s office. This glossary covers every term you’ll see next to a result from the baby percentile calculator, in plain English.

Percentile

The percentage of same-age, same-sex children in the reference population who measure less than the child being evaluated. A weight-for-age percentile of 30 means 30% of same-age, same-sex children weigh less, and 70% weigh more. Percentiles run from just above 0 to just below 100, and the 50th percentile is always the exact median — not a “passing” or “target” score.

Z-score (standard score)

The number of standard deviations a measurement sits from the reference median. A Z-score of 0 is exactly average; +1 is one standard deviation above the median; -2 is two standard deviations below. Clinicians often track Z-scores instead of percentiles at the extreme ends of the chart, because percentile has almost no room left to move between, say, the 0.1st and 0.01st percentile, while the Z-score keeps changing in a way that’s easier to plot over time.

LMS parameters (L, M, S)

The three published values — L (skewness/Box-Cox power), M (median), and S (coefficient of variation) — that WHO and the CDC release for every age and sex. Together they describe the full shape of the growth distribution at that exact age, which is what lets a calculator convert any raw measurement into an exact Z-score and percentile rather than reading it off a printed chart by eye. See the baby percentile calculator methodology for the full LMS formula.

WHO Child Growth Standards

The growth reference used for children under 24 months in US clinical practice. Built from healthy, predominantly breastfed infants across six countries, it describes how children should grow under optimal feeding and health conditions — a standard, not just an average.

CDC 2000 Growth Charts

The growth reference used for children 2 through 20 years old. Built from a large sample of real US children measured between the 1960s and 1990s, it describes how children actually grew in that population — a reference, not a target.

SGA / AGA / LGA

Terms used mainly at birth: Small for Gestational Age (birth weight below the 10th percentile for gestational age), Appropriate for Gestational Age (10th-90th percentile), and Large for Gestational Age (above the 90th percentile). These use birth-specific intrauterine growth curves, not the standard infant growth charts — a separate reference from the one this calculator uses for ongoing growth tracking after birth.

Corrected (adjusted) age

For babies born before 37 weeks, age counted from the original due date rather than the birth date. Used for growth and developmental tracking through roughly 24 months. Plugging a premature baby’s chronological (birth) age into any growth percentile calculator, including this one, will understate how well they’re actually growing. See our adjusted age calculator and the premature and NICU parents’ guide for the full explanation.

Catch-up growth

The period after a difficult start (prematurity, illness, feeding difficulty) when a baby’s growth rate temporarily exceeds the typical rate for their age, moving them back up toward their genetically expected percentile band. Catch-up growth is expected and healthy in this context — it looks like a rising percentile trend across visits rather than a single high reading.

Failure to thrive (growth faltering)

A clinical concern, not a single number — generally used when a child’s weight-for-age or weight-for-length percentile crosses down through two or more major percentile lines on the growth chart over time, or falls and stays below the 2nd-5th percentile alongside other clinical signs. A single low reading is not, by itself, failure to thrive; the trend across visits is what matters. See common growth percentile mistakes for why single readings get over-interpreted.

Recumbent length vs. standing height

Recumbent length is measured with the child lying flat, used for children under 2. Standing height is measured upright, used for 2 and older. The two aren’t perfectly interchangeable — standing height typically reads 0.5-1 cm shorter than recumbent length at the same true height because of spinal compression under gravity, which is why the reference charts (and this calculator) switch measurement method at the same age they switch data sets.

Ready to convert a real measurement into a percentile and Z-score? Use the baby percentile calculator, or see growth percentile examples by age for the math worked out on real numbers.