All infants, including preterm infants should be expected to grow.
Some preterm infants will remain smaller than full-term, healthy infants, even in the face of adequate nutrition and acceptable growth rates.
Growth Expectations
Catch up Growth in Preterm Infants
Failure to Thrive
Growth Expectations:
Expectations for optimal growth of the preterm and low birth weight infant are not precisely known. It is clear that this population is not homogenous in regards to growth. During hospitalization, in utero rates of growth (15 g/kg/d weight gain, 0.5-1.0 cm/wk length) are often used as established goals. After a period of initial growth delay in the early postnatal period (1-3 weeks) many preterm infants achieve this rate of growth. However, infants with numerous medical complications during hospitalization may have periods of growth delays related to the medical/nutritional consequences of their illnesses. Clark et al identifies postnatal growth restriction as a problem in preterm infants.
It is unclear what represents optimal growth for the preterm infant after hospital discharge. Preterm infants have been followed post-discharge up to school age and their patterns of growth have been described. Over time, changes in survival and treatment for very low birth weight (VLBW) infants make application of the results of these studies to current infants problematic. In general, infants with chronic medical conditions may not exhibit "catch-up growth" until early childhood or school age, and some former preterm infants will remain small throughout life.
Hack et al., 1996 compared rates of subnormal weight and height in childhood in a cohort of VLBW infants. Although growth parameters for the group were consistently lower than those of a full term cohort, rates of subnormal growth in the VLBW cohort fell at each measurement point from birth through 8 years of age. At 40 weeks gestational age, 54% of appropriate for gestational age (AGA) and 82.5% of small for gestational age (SGA), infants were less than 2 standard deviations from the mean for height. At 20 months, 12% of the AGA children and 30% of the SGA children were below the 3rd percentile for height, and by eight years of age, only 6% of the AGA and 18% of the SGA children were below the 3rd percentile.
Adult height may be less in those who were born VLBW. Powls et al., 1996 reported that in early adolescence, VLBW infants were smaller than normal birth weight peers. This difference remained when parental heights were taken into account. Seven percent of this cohort of 137 VLBW infants were below the 3rd percentile for height and 22.5% were below the 10th percentile for height. Projected adult heights of this cohort were less than genetic potential would indicate.
See other studies by Casey, Friel, and Georgieff.
Other studies suggest that many preterm infants will exhibit growth comparable to term infants and show catch-up growth in the first 3 years. Saigel and Hirata both demonstrated "catch-up potential" in very low birth weight infants during adolescence.
Growth rates and growth potential are influenced by genetic as well as environmental factors. Prenatal and postnatal variables will modify the growth patterns observed in individual infants. Infants who experience severe in utero growth retardation, repeated illness, and chronic medical conditions may remain smaller than their term cohorts beyond 3 years of age. However, these infants may be able to achieve rates of growth that are comparable to their term cohorts. See incremental growth expectations based on CDC estimates compared with preterm growth expectations.
Catch up Growth in Preterm Infants:
The classic use of the term "catch-up growth" implies an infant or child who demonstrates accelerated rates of growth following a period of growth failure. It is important to note that the term appears in some literature and research reports in a non-classical sense with respect to preterm infants and identify those infants who reach >10th percentile in growth parameters. Failure to exhibit this kind of catch-up growth may not be interpreted in the same manner as an infant who fails to gain in weight or length at "normal" rates. The infant who continues to gain 20-30 g/d after 6 months of age may be described as showing catch-up growth in the sense that the infant is gaining at rates that exceed the 50th percentile rate of weight gain for a term infant.
Failure to Thrive:
Preterm and low birth weight infants exhibit increased morbidity and mortality when compared to term infants. Kelleher, et al., 1993 identified a 21% incidence of failure to thrive in VLBW within the first 36 months of life. The prevalence of failure to thrive (FTT) peaked between 4-8 months of age in this study. The risk for increased morbidity and mortality is inversely related to gestational age and size. These infants also exhibit different patterns of growth than term, non-LBW infants. Alterations in growth patterns, FTT particularly, may be one of the morbidities associated with decreasing gestational age and birth weight.
The term FTT, in its classic sense, refers to failure to gain in weight and length at expected rates. Preterm infants are at risk for being misidentified as FTT when the term is applied in other ways (i.e., weight or length <5th percentile, without consideration of growth velocity). Extremely low birth weight (ELBW) infants and infants with severe intrauterine growth retardation (IUGR) may demonstrate periods of accelerated rates of weight gain and remain <10th percentile in weight and length for several years.
Nutritional practices have been associated with growth outcomes (catch-up growth and failure to thrive). Although discharge practices are currently changing rapidly, in the past it was usual practice to discharge preterm infants when they weighed approximately 2 kg, were nippling all feedings, and were able to maintain their temperature outside an isolette. Prior to this transition home, many of these infants were transitioned to breastmilk or standard infant formula at an energy density of 20 kcal/oz. Several studies have demonstrated improved growth rates after hospital discharge in infants fed human milk or formula that was energy and nutrient dense after hospitalization. These studies suggest that continued intake of energy-dense feedings may support continuation of growth rates established in hospital. In addition some VLBW and ELBW infants may have higher mineral needs (calcium, phosphorus, and possibly zinc) for optimal growth after hospitalization. See sections on Infant Formulas and Nutrition Supplement Recommendations for more information.
Ernst et al., 1990, identified practices that were associated with poor growth outcomes in a group of VLBW infants. These practices largely included inappropriate feeding transitions during the first year of life (introduction of solids before 6 months corrected age), introduction of cow's milk before 12 months corrected age, and use of low-fat milks.