Growth charts are fundamental tools for monitoring the health and development of young children. For babies born prematurely (before 37 weeks of gestation), a specialized approach is needed to track their growth accurately. The World Health Organization (WHO) provides a standard reference that assesses a preterm infant’s progress. These WHO charts help healthcare providers and parents visualize whether a baby is gaining weight, growing in length, and developing a head circumference at a rate suggesting optimal health. Using this tool requires understanding its design principles, especially the adjustment made for the time the baby spent developing in the womb.
Establishing the International Standard
The WHO developed its Child Growth Standards to describe how healthy children should grow in optimal environments worldwide. These standards are prescriptive, meaning they show the growth pattern of healthy, breastfed infants from diverse populations, establishing a global reference. The WHO’s methodology was adopted by the INTERGROWTH-21st project to create specific international standards for preterm infants. This ensures that the chart does not just reflect the growth of babies in a single region or under specific, non-ideal conditions.
The goal was to move away from older, localized charts that were often based on populations whose growth may have been constrained by factors like poor nutrition. By using a multi-ethnic, standardized approach, the WHO-aligned charts establish a robust benchmark for healthy growth trajectories. This international standard helps clinicians globally compare a preterm baby’s growth against a gold standard of health.
The Critical Role of Corrected Age
The most significant modification when using the WHO standards for a preterm infant is the application of “corrected age,” also known as adjusted age. Chronological age is simply the time elapsed since the baby’s birth, but for a premature baby, this metric is misleading for growth and developmental assessment. A baby born eight weeks early has missed eight weeks of in-utero development that a full-term baby experienced.
Corrected age accounts for this difference by calculating the baby’s age from the mother’s original due date, not the actual birth date. The calculation is straightforward: you subtract the number of weeks the baby was born prematurely from their chronological age. For example, a baby who is 20 weeks old (chronological age) but was born 10 weeks early has a corrected age of 10 weeks.
Plotting a preterm baby’s measurements on the chart using their chronological age would place them far below the typical growth lines, making their growth appear suboptimal. Using the corrected age ensures the baby is compared to full-term infants who are at the same stage of development. This adjustment is generally continued until the child is around two to three years old, by which time most premature infants have achieved “catch-up growth.”
Interpreting the Growth Curve
The WHO preterm chart tracks three main physical measurements: weight, length (or height), and head circumference. These measurements are taken by a healthcare professional and then plotted against the baby’s corrected age on the corresponding gender-specific chart. The chart features several curved lines, which are percentile lines, typically ranging from the 3rd to the 97th percentile.
These percentile lines represent the distribution of measurements within the healthy reference population. For instance, if a baby’s weight falls on the 50th percentile line, it means that 50% of healthy babies of the same age and sex weigh less, and 50% weigh more. The goal of plotting is not to hit a specific percentile, but to ensure the baby’s measurements consistently follow one of these curves over time.
A consistent trend, such as staying near the 10th or 50th percentile, indicates predictable and healthy growth. However, a sudden, significant shift—such as crossing two or more major percentile lines either up or down—can signal a change in health status or nutrition that warrants further investigation. This movement provides the most valuable clinical information.
Using the Chart for Ongoing Health Assessment
Consistent and accurate plotting on the WHO preterm chart provides a dynamic record that guides clinical decision-making. The chart is instrumental in identifying “failure to thrive,” which is defined not by a single low measurement but by a growth pattern that falls below expected velocity. If a baby’s weight gain velocity slows down or drops off a curve, it prompts a review of their nutritional intake.
The chart helps healthcare teams manage the period of “catch-up growth,” which is the rapid growth phase many preterm infants experience to bring them closer to the size of their term-born peers. Monitoring this period is delicate; while catch-up is desired, excessive or too-rapid weight gain can potentially increase the risk for later metabolic issues. The chart helps guide precise nutritional adjustments, such as increasing the caloric density of feeds.
Tracking head circumference is particularly important as it is a proxy for brain growth and neurological development. A disproportionate drop in head circumference percentile relative to weight or length can be a serious sign requiring immediate medical attention. Ultimately, the WHO preterm chart serves as a visual predictor, helping professionals assess current health and long-term developmental trajectories.