A baby is considered premature if born before 37 completed weeks of gestation, earlier than a standard 40-week pregnancy. These infants typically have smaller measurements for length, weight, and head circumference at birth compared to their full-term counterparts. Whether this initial size difference results in shorter adult height is complex, depending less on the prematurity itself and more on the baby’s health and growth trajectory after birth. Many premature babies are initially smaller, but their subsequent growth is highly variable and often subject to rapid compensatory growth.
Initial Size and the Concept of Corrected Age
A premature infant’s length is initially measured against specialized growth charts, such as the Fenton curves. These charts track growth based on gestational age, mimicking development in the womb, and are used until the baby reaches the full-term due date. Using a standard chart immediately after birth would incorrectly suggest growth failure, as preemies are weeks or months behind in development.
To accurately assess a premature baby’s development and growth trajectory, healthcare providers use “corrected age,” also known as adjusted age. This is calculated by subtracting the number of weeks the baby was born early from their chronological age. For example, a six-month-old born two months early has a corrected age of four months. This means their growth and developmental milestones are benchmarked against a four-month-old full-term infant. This correction is typically applied until the child is about two years old, providing a fairer assessment of postnatal progress.
The Process of Catch-Up Growth
Catch-up growth is a biological phenomenon where a premature infant experiences accelerated growth after birth to minimize the size difference with full-term peers. This rapid growth is a compensatory mechanism, often beginning shortly after hospital discharge. The most significant period of catch-up usually occurs within the first 12 to 18 months of life, though it can continue up to three years of age.
Most healthy premature babies, particularly those born moderately premature and appropriate in size for their gestational age, successfully complete this phase. By the age of two, the majority of these children follow the standard growth curves. Successful catch-up growth means the child reaches a height percentile consistent with their genetic potential, reducing the likelihood of short stature later in life.
Factors Influencing Persistent Height Deficits
While many preemies catch up, a subset remains shorter throughout childhood and into adulthood due to specific medical or prenatal conditions. The most significant factor inhibiting successful catch-up growth is Intrauterine Growth Restriction (IUGR), meaning the baby was born small for gestational age (SGA). IUGR suggests the baby’s growth was already restricted in the womb, and it is a stronger predictor of persistent short stature than prematurity alone.
Severe Prematurity and Chronic Illness
Being born at an extremely low gestational age, often less than 28 weeks, also increases the risk of persistent height deficits. Severe medical complications that occur after birth, such as chronic lung disease like Bronchopulmonary Dysplasia (BPD), can impede growth. BPD increases the infant’s metabolic demands while simultaneously making feeding and nutrient absorption difficult, leading to extrauterine growth restriction.
Only about 10% of children born SGA fail to achieve catch-up growth and remain short as adults. These conditions—IUGR, severe prematurity, and chronic illness—disrupt the delicate balance of hormones and nutrients required for bone development and linear growth. This failure to normalize height during the first few years of life results in a permanent shift to a lower growth trajectory.
Clinical Monitoring and Interventions for Growth
Pediatricians and specialists closely monitor the growth of former premature infants using the corrected age and tracking them on standard World Health Organization (WHO) or Centers for Disease Control (CDC) growth charts. A consistent downward trend in a child’s growth curve, or a failure to maintain the expected trajectory, may prompt a more in-depth evaluation.
Nutritional Support
Early intervention focuses on optimizing nutrition, which is the most effective strategy for encouraging catch-up growth. Nutritional interventions include fortifying breast milk or using high-calorie formulas to ensure the infant receives sufficient energy and protein for rapid growth.
For children who continue to show persistent growth failure, specifically those born SGA who have not achieved catch-up growth by two years of age, Growth Hormone (GH) therapy may be considered. This treatment is reserved for cases where poor growth is documented and other causes have been ruled out, aiming to improve final adult height for those who missed their early growth window.