The Journal of Pediatrics
Volume 154, Issue 4 , Pages 473-475, April 2009

Postnatal Growth in Preterm Infants: Too Small, Too Big, or Just Right?

  • Lewis P. Rubin, MD

      Affiliations

    • Corresponding Author InformationReprint requests: Lewis P. Rubin, MD, University of South Florida, Department of Pediatrics, STC 5th Floor, Room 5091, 2A Columbia Dr, Tampa, FL 33606

Department of Pediatrics, Division of Neonatology, University of South Florida College of Medicine, Tampa, Florida

Article Outline

Abbreviations: ELBW, Extremely low birth weight, NICU, Neonatal intensive care unit, SGA, Small for gestational age, VLBW, Very low birth weight

 

A persistent challenge throughout the history of preterm infant care is sustaining “appropriate” extrauterine growth. Optimizing nutrition and growth for this population is fraught with difficulties stemming from severe illness, digestive organ immaturity, lack of consensus about proper reference data, and insufficient information on nutrient accrual. Inevitably, different practitioners hold different, sometimes contradictory, attitudes about nutritional strategies in prematurity.

See related article, p 486

During the 1990s, improved survival for very low birth weight (VLBW, 401-1500 g) and especially for extremely low birth weight (ELBW, <1000 g) infants has underscored the high frequency of postnatal growth failure. In 2001, the NICHD Neonatal Research Network reported that 97% of all VLBW infants and 99% of ELBW infants had weights <10th percentile at 36 weeks postmenstrual age.1 Several investigators have concluded that postnatal growth restriction currently is to be anticipated for many extremely preterm infants by hospital discharge.2, 3, 4

In this issue of The Journal, Olsen et al5 have evaluated use of the Ponderal index (weight-for-length, g/cm3) as a complementary measure of growth of preterm infants in the neonatal intensive care unit (NICU). Rohrer's Ponderal index6 essentially is an indirect measure of soft tissue and, inferentially, of fat accumulation. The authors' aim is to incorporate body proportionality with weight-for-age in routine assessment of growth in this high-risk preterm population.

Why is this important? Adiposity and body mass index (weight/height2) have become an increasingly important part of health assessment. Olsen et al5 astutely observe that prematurity is the only time during the lifecycle in which body proportionality is not routinely assessed.

They evaluated 1214 infants 26 to 29 weeks at birth, included in a registry database (1991–2003), who had weight and length recorded at birth and discharge. This data set constitutes a secondary analysis of the Cincinnati site in the NICHD Neonatal Research Network. The authors use the common definitions of “small” (SGA), “appropriate,” and “large” as, respectively, <10th, 10th to 90th, and >90th weight percentile, originally derived from the Lubchenco growth chart.7, 8, 9 Their use of the Lubchenco appropriate and large for gestational age weight cutoffs is acknowledged as a convenience. The newer Riddle growth chart 9 better fits the Cincinnati preterm study sample, but that grid does not include a weight-for-length curve. Despite these significant limitations, the finding of Olsen et al5 advance the field of neonatal growth and nutrition. They found significant discordances at hospital discharge between the weight-for-age and weight-for-length methods of growth assessment. The overall percent of infants classified as SGA almost doubled from birth (12%) to discharge (21%). In contrast, large-for-length (increased Ponderal Index) infants increased from 5% to 17%, possibly indicating excessive adiposity. These findings support the common observation that when ELBW infants finally reach NICU discharge, many are small but plump.

It is increasingly appreciated that either undernutrition or overnutrition may have deleterious health consequences, especially when they occur during critical developmental windows, that is, the “developmental origins of adult disease” theory.10, 11 Recent data suggest that growth velocity during ELBW infants' NICU course exerts a significant, possibly independent, effect on neurodevelopmental and growth outcomes at 18 to 22 months corrected age.12 “Overnutrition” in SGA infants may increase risk for hypertension and cardiovascular disease.13 Randomized intervention trials of early nutrition with long-term follow-up are beginning to show that nutrition in early life has a major impact on health into early adulthood, notably on cardiovascular disease risk, bone health, and cognitive function.11

Advancement in this field presupposes available high-quality and consistent postnatal growth data. To a great extent, VLBW infant growth assessment depends on which growth chart is chosen. An unpublished survey of 118 neonatal health professionals cited by Fenton14 found the most frequently used NICU growth charts were those of Babson15 (50%), Lubchenco8 (18%) and Dancis.16 Continued use of these grids in assessing preterm infants is fraught with several methodologic problems that include small numbers of extremely preterm infants, changes in NICU care occurring over several decades (The newest of these grids is more than 30 years old.), errors in attributing gestational age, and inadequate modeling techniques, such as an assumption that weight data conform to modeling as Gaussian distributions.

Fenton14 updated the Babson chart with a meta-analysis of several recent data sets. The chart is not based solely on longitudinal data and, therefore, does not represent the actual growth of preterm infants. A significant advance has been publication of a NICHD longitudinal growth grid for 100-g birth weight intervals from 500 to 1500 g.17 In effect, this chart, based on a large, multicenter, prospective cohort, describes VLBW postnatal growth reflecting current imperfect and disparate nutritional practices. But, by design, it represents “things as they are,” not necessarily a prescriptive for an “ideal” growth.

The different growth grids and curves, important as they have been for normative data and possible benchmarking, also present several conceptual problems. For one, prevalence of SGA varies markedly, depending on the reference grid used. Second, cross-sectional data on the basis of birth weight of newborns delivered at different gestations7, 12, 18, 19, 20 are not physiologically appropriate references for preterm infants who have experienced extrauterine growth and development for weeks to months. Weight changes during the perinatal transition following birth primarily reflect fluctuation of total body water, whereas later changes reflect growth responses to nutritional management.21 Additionally, grids indicating solely percentiles14, 19, 20 rather than z scores22 support thinking about a continuous variable (weight) as discrete categories.

Empirical growth assessments with the large NICHD Neonatal Research Network database, such as those of Ehrenkrantz et al12, 17 and Olsen et al5 help characterize postnatal growth of extremely preterm infants who have been managed with recent nutritional practices. Several important questions confront us. What are the roles of breast milk as the base for the ELBW diet? During this decade, more “aggressive” and early nutrition is being promoted as a preventive to endemic poor growth in ELBW infants.23 The alternative question arises, “Are we overcompensating for these infants' significant nutritional deficits in the first weeks of life?”2, 24

As Olsen et al5 point out, the observed shift of small-for-length and appropriate-for-length preterm infants at birth to large-for-length infants may be a result of accelerated weight gain, poor linear growth, or both. The paucity of data about what might represent physiological growth and body composition calls for innovative, simple, and noninvasive infant body composition assessments.

Validated measures of percentage of in vivo body fat in preterm infants include MRI multi-compartment isotope dilution techniques,25 dual-energy X-ray absorptiometry,26 and nonradioactive air displacement plethysmography.25, 27 The available air displacement plethysmograph for infants from 1 to 8 kg (PEA POD, Life Measurement Inc., Concord, California) is currently in clinical trials for use in nutritional assessment and therapy.

In the end, improved information about body composition and proportionality should define health promoting and deleterious metabolic consequences of different preterm postnatal growth patterns. That information should lead to reassessment of the longstanding American Academy of Pediatrics recommendations that growth of preterm infants should replicate that of fetuses of the same gestational age.28

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References 

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 Lewis P. Rubin, MD, serves as a scientific advisor for Abbott Labs and Life Measurement, Inc.

PII: S0022-3476(08)01148-7

doi:10.1016/j.jpeds.2008.12.041

Refers to article:

  • Use of a Body Proportionality Index for Growth Assessment of Preterm Infants , 28 November 2008

    Irene E. Olsen, M. Louise Lawson, Jareen Meinzen-Derr, Amy L. Sapsford, Kurt R. Schibler, Edward F. Donovan, Ardythe L. Morrow
    The Journal of Pediatrics April 2009 (Vol. 154, Issue 4, Pages 486-491)

The Journal of Pediatrics
Volume 154, Issue 4 , Pages 473-475, April 2009

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