A baby is considered premature, or a “preemie,” when born before completing 37 weeks of gestation. This early arrival interrupts the natural course of development, leading to differences compared to full-term infants, with growth being a primary concern for parents. While an initial period of slower growth is expected and medically managed, the ultimate goal is for the child to achieve a growth trajectory that aligns with their potential. This journey involves dedicated monitoring, tailored nutrition, and understanding the unique biological challenges prematurity presents.
Initial Growth Trajectory and Medical Factors
Preemies typically experience an initial period of slower growth than full-term counterparts because they missed a crucial phase of in-utero development. The third trimester is when a fetus undergoes its most rapid growth, accumulating fat stores and completing organ system development. Babies born early miss this period, resulting in low energy reserves and a reduced ability to process nutrients effectively after birth.
This initial deceleration in growth is often termed extrauterine growth restriction, driven by biological and environmental factors. Energy demands placed on a premature infant in the neonatal intensive care unit (NICU) are significantly higher than they would be in the womb. Maintaining body temperature, fighting infections, and recovering from medical complications like respiratory distress syndrome divert energy away from growth. Minimal energy reserves combined with high caloric expenditure create a barrier to immediate weight gain.
Furthermore, the organs responsible for nutrient absorption, particularly the gastrointestinal tract, are still immature. This immaturity limits the amount of nutrients a preemie can safely tolerate and absorb from feedings. Initial growth goals are often set to match the rate of the third-trimester fetus, approximately 15 to 20 grams per kilogram of body weight per day. Achieving this rate requires a delicate balance of providing aggressive nutritional support while respecting the limitations of the baby’s developing systems.
Tracking Development Using Corrected Age
To accurately monitor a preemie’s progress, healthcare providers use a calculation known as the “corrected age” or “adjusted age.” Chronological age, which is simply the time since birth, does not account for the developmental time lost by being born early. The corrected age provides a more realistic benchmark for measuring physical growth and developmental milestones.
The calculation is straightforward: the number of weeks the baby was born prematurely is subtracted from their chronological age. For example, a baby born 12 weeks early who is now 6 months old (24 weeks) has a corrected age of 12 weeks (three months). This means the baby’s growth and development should be compared to that of an average three-month-old, not a six-month-old.
Using corrected age is essential because standard growth charts designed for full-term babies would make a preemie consistently appear behind. Specialized growth charts, such as the Fenton preterm growth chart, are often used initially. The corrected age allows providers to plot a preemie’s weight, length, and head circumference against an appropriate population. Physicians continue to use the corrected age for tracking developmental milestones, such as sitting up and walking, typically until the child reaches two to three years of age.
Specialized Nutritional Support for Catch-Up Growth
The primary medical intervention for promoting accelerated growth, known as “catch-up growth,” is a specialized nutritional regimen. Since premature infants have high metabolic demands, their diet must be significantly denser in calories, protein, and micronutrients than that of a full-term baby. This is achieved through fortifying human breast milk or utilizing specialized preterm formulas.
Human milk fortifiers (HMF) are supplements added directly to the mother’s expressed breast milk, increasing the protein, mineral, and vitamin content without significantly increasing the volume. This fortification is necessary because unfortified human milk, while optimal for term babies, lacks sufficient protein and calcium to support a preemie’s rapid growth needs. The goal is to provide a high protein intake, often targeting 3.5 to 4.0 grams per kilogram of body weight per day, to promote the growth of lean body mass.
Specialized premature formulas are designed to be nutrient-dense, typically providing 24 kilocalories per ounce, compared to the standard 20 kilocalories per ounce found in term formula. These formulas feature a higher protein-to-energy ratio and include specific fats, like medium-chain triglycerides (MCTs), which are easier for the immature digestive system to absorb. This nutritional strategy is maintained until catch-up growth is firmly established, usually for the first year of life, to close the growth gap and improve long-term cognitive outcomes.
Long-Term Growth Outcomes and Final Stature
The success of the initial catch-up growth period largely determines the long-term growth outcomes and final adult stature of a former preemie. For most infants born moderately preterm, the period of rapid catch-up typically concludes by the age of two or three. By this time, the majority of these children will have achieved a weight and height that falls within the normal range for their chronological age.
However, the likelihood of remaining smaller than average is higher for infants born extremely preterm (before 28 weeks of gestation) or those who were small for gestational age (SGA). Children who experienced significant neonatal illnesses or had difficulty achieving adequate nutrition in the first few months are also at greater risk for persistent growth restriction. These factors can impact the final adult height, sometimes resulting in a measurable, though usually small, average height deficit compared to their full-term peers.
The trajectory of growth is often more important than the exact percentile. Preemies who follow a consistent, healthy growth curve and receive appropriate nutritional support generally reach their genetically programmed height potential. While the initial growth phase is slow and requires specialized attention, the majority of children born prematurely overcome this early disadvantage and continue to grow and develop without significant difference from their peers.