The World Health Organization (WHO) Preterm Growth Standard is an evidence-based tool developed to monitor the growth and nutritional progress of infants born prematurely (before 37 completed weeks of gestation). This chart provides a systematic, globally relevant benchmark for assessing whether a preterm baby is growing optimally. By tracking physical measurements over time, healthcare providers use this chart to make informed decisions about nutrition and developmental support. The chart’s design reflects the unique physiological needs and growth patterns of this vulnerable population.
The Need for Specialized Preterm Monitoring
A premature infant’s growth trajectory differs fundamentally from that of a baby born at full term, making standard growth charts inaccurate for this group. Charts based on healthy, term-born infants reflect a pattern of growth achieved in the final weeks of gestation and beyond, which an early infant’s body cannot immediately replicate. Using a standard chart would incorrectly suggest that nearly all premature babies are severely underweight and undersized simply because they were born early.
The physiological immaturity of preterm babies means their initial growth outside the womb often requires a transitional phase that attempts to mimic the rapid, nutrient-driven growth that would have occurred in utero. Specialized monitoring is necessary because standard charts fail to account for the dramatic differences in body composition and organ development present at birth. This ensures the infant is meeting size expectations relative to their gestational age and not their chronological age. Without this specialized approach, there is a risk of misclassifying a healthy, growing preterm baby as having growth faltering, or conversely, missing a true nutritional deficit.
Understanding the WHO Preterm Growth Standard
The WHO Preterm Growth Standard assesses physical growth using three anthropometric measurements: weight-for-age, length-for-age, and head circumference-for-age. This multi-parameter approach allows clinicians to monitor overall size, proportionality, and brain growth, which is a significant indicator of neurodevelopmental health.
The standard utilizes a dual-component design to bridge the gap between fetal life and independent postnatal growth. For the period immediately following birth until the expected due date (term equivalent age), specialized reference curves are used that reflect the growth rate of a healthy fetus. Once the infant reaches the term equivalent age, their measurements are plotted onto the main WHO Child Growth Standards, adjusted for prematurity.
Growth is assessed by plotting measurements against percentile curves or Z-scores. Z-scores offer a precise statistical measure, indicating how many standard deviations an infant’s measurement is away from the median. Values between -2 and +2 are generally considered within the typical range. The use of Z-scores is particularly helpful for preterm infants because it allows for a more sensitive tracking of growth velocity and subtle changes.
The Practical Application of Corrected Age
Accurately plotting a preterm infant’s growth requires the consistent application of corrected age, sometimes called adjusted age. Corrected age is the baby’s chronological age (age since birth) minus the number of weeks they were born prematurely. This calculation adjusts the baby’s age to reflect the time they would have spent in the womb had they been born at 40 weeks gestation.
To determine the corrected age, calculate the degree of prematurity by subtracting the infant’s gestational age at birth from 40 weeks. For example, if a baby was born at 32 weeks, they were 8 weeks premature. If that baby is now 6 months old chronologically, the corrected age is 4 months.
The growth measurements are plotted on the chart corresponding to this corrected age, not the actual birth date. This is done because a preterm infant is expected to meet the developmental and growth milestones of an infant who is the same age as their corrected age. Health professionals generally continue to use corrected age for growth monitoring until the child reaches approximately 24 months of chronological age, though some may extend this to 36 months, depending on the degree of prematurity. Failing to use this correction factor would result in the infant’s measurements plotting dramatically low, giving a false impression of severe growth failure.
Interpreting Growth Trends
Interpreting the plotted data involves looking at the direction and speed of the line connecting the measurements, known as growth velocity. A single measurement point offers a snapshot of size, but the trend line over several visits reveals the pattern and health of the infant’s growth. The goal for many preterm infants is to exhibit “catch-up growth,” a period of rapid, accelerated weight and length gain allowing them to approach the percentiles of their term-born peers.
During this catch-up phase, it is normal for a baby’s plot line to cross percentile lines in an upward direction, a pattern that is expected for a premature infant. Conversely, a trend line that flattens out or shows a sharp drop, particularly falling below the 3rd or 5th percentile, indicates growth faltering or a nutritional issue. This requires immediate attention and evaluation.
The head circumference measurement is significant because it is a proxy for brain growth. A consistently rising head circumference trend, even if other measurements are lagging, is a reassuring sign of healthy neurological development. Any sudden deceleration or acceleration in head growth velocity warrants further medical investigation.