KEY POINTS
- 1.
All infants should be classified at birth with the use of appropriate anthropometric measurement techniques to determine risk status for complications such as hypoglycemia or catabolism. Growth charts and tools to determine z -scores are readily available and should be used.
- 2.
Hospitalized infants are at higher risk for growth impairment, so patterns on a growth chart and growth velocity throughout hospitalization should be routinely monitored as part of standard clinical care.
- 3.
A nutrition-focused physical examination includes anthropometric assessments, vital signs, and careful assessment for any evidence of nutritional deficiencies.
- 4.
Controversy exists regarding frequency and timing of biochemical monitoring for common complications of prematurity, such as metabolic bone disease and cholestasis. Clinicians should consider adopting a standardized approach that incorporates expert opinion when evidence is lacking.
- 5.
All high-risk infants should have daily assessment of nutrient intake.
- 6.
A registered dietitian should be part of the healthcare team to assess neonatal growth and nutrient intakes.
Introduction
Growth is a normal state for infants and an indicator of wellness. All infants deserve personalized nutritional care that will promote growth and a healthy start to life. Nutritional assessment based on growth history; biochemical, clinical, and physical parameters; and nutritional intake can allow the clinician to determine which infants are not growing well and/or have not achieved adequate nutrition.
Preterm and critically ill infants need focused growth monitoring, because nutrient needs are not based on the cues from the infant but have to be estimated and delivered by the medical team. In this chapter, we seek to describe clinically available tools and the highest-quality evidence available to assess neonatal nutritional status. Further information on parenteral and enteral nutrition is included in separate chapters focused on nutrition requirements.
Classification of the Newborn
Newborn assessment and anthropometric classification at the time of delivery reflect intrauterine growth and allow identification of infants who are at higher nutrition risk. Many of these small- and large-for-gestational age (SGA and LGA) infants are also at risk of early metabolic complications such as hypoglycemia in the first days of life. SGA infants often have low glycogen stores, whereas many LGA infants may be at risk of abnormal glucose levels due to maladaptation to higher glucose loads from uncontrolled maternal diabetes. Symptoms of hypoglycemia include jitteriness, poor feeding, tachypnea, floppy tone, and even seizures. In subsequent days, these early metabolic abnormalities may resolve, but multiple studies have shown these infants to also be at risk of abnormal growth trajectories ( Chapter 19 , 20 ).
Birth Weight Classification
- •
Per Centers for Disease Control and Prevention 2019 data, about 8.3% of all infants have a birth weight of less than 2500 g and are categorized as low birth weight (LBW). , Those born with a birth weight of less than 1500 g are identified as very low birth weight (VLBW) infants, which is about 1.4% of all infants. Less than 1% of infants are born with a weight of less than 1000 g and are categorized as extremely low birth weight (ELBW).
- •
Birth weight may reflect the adequacy of nutritional stores, such as those of protein, fat, iron, calcium, phosphorus, and other important nutrients.
- •
Higher gestational age reflects maturity with better ability to tolerate fluctuations in biochemical levels of various nutrients. Adjusting age for prematurity assists in setting expectations for developmental achievements.
- •
Premature birth is the most common cause of low birth weight. We have nutritional guidelines for VLBW, preterm infants that aim to meet the metabolic and growth needs and achieve growth rates similar to those in utero. ,
- •
Besides prematurity, another important cause of low birth weight is fetal or intrauterine growth restriction (IUGR). At any gestation, infants may be classified as SGA (birth weight < the 10th percentile), appropriate for gestational age (AGA), or LGA (birth weight > the 90th percentile). Based on these definitions, 20% of the population is expected to have a birth weight outside the AGA category. Not all infants who meet criteria to be labeled SGA or LGA are medically compromised, nor have they suffered from an obvious intrauterine insult that may explain their abnormal growth. Some conditions are definitely more frequent in the LGA and SGA categories, but it is important to understand that these definitions are statistical and somewhat arbitrary, and therefore, not everyone in these categories has ongoing pathology. Many of these infants have weights that are normal, healthy metrics for their genetic profiles. In some infants, IUGR may also be due to maternal factors that did not support in utero growth at their genetic potential. These infants also may not have intrinsic abnormalities.
- •
Newer technologies can often allow the identification of the cause for IUGR. Many fetuses are growth restricted because of compromised fetal blood flow; the Doppler waveform of blood flow in the umbilical vessels can provide useful information. , Similarly, magnetic resonance imaging of whole-body fetal adipose tissue can also provide useful information to aid assessment of fetal growth and placental sufficiency.
Features of Fetal Growth Restriction
- •
Can be identified via prenatal ultrasound or by physical exam at birth
- •
Thin, wasted appearance on prenatal imaging and after birth
- •
Deficiency of subcutaneous tissue and muscle, noted in the cheeks, neck and chin, arms, back, buttocks, legs, and trunk
- •
Cranial sutures may be widened, and the umbilical cord may be thin and lacking in Wharton’s jelly
- •
Dysmorphic features could indicate a congenital syndrome
One scoring system is the Clinical Assessment of Nutritional Status (CANS) ( Fig. 22.1 ). The intent of this exam is to distinguish a term infant who suffered from fetal malnutrition from an infant who is simply physiologically small for gestational age. This malnutrition might result in altered body composition and impaired neurodevelopmental potential. The signs of malnutrition include:
- •
Hair: silky versus straight
- •
“Staring” or flag sign
- •
Reduced buccal fat in the cheeks
- •
Sharply defined thin chin or fat double chin
- •
A thin, clearly evident neck with loose, wrinkled skin
- •
“Accordion” pleating of the skin of arms and legs with loose, easily lifted skin over major joints
- •
Loss of subcutaneous fat on the back with skin easily lifted
- •
Minimal fat and wrinkled skin over abdomen
- •
Buttocks with deep folds
This CANS simple scoring system was developed for term infants and is intended to be used in the first 48 hours of life. , Some researchers then applied the CANSCORE to preterm infants. They found that maternal hypertension and preeclampsia, oligohydramnios, disturbed umbilical artery Doppler flow, neonatal hypoglycemia, polycythemia, feeding intolerance, and necrotizing enterocolitis were all associated with what was described as fetal malnutrition. Interestingly, not all of the infants who were identified as having had fetal malnutrition were classified as SGA based on anthropometrics. We clearly need ways for more accurate identification of the predisposing factors and validation of the CANSCORE for preterm infants.
LGA infants can be born to constitutionally tall parents or to mothers with uncontrolled diabetes or obesity. , Infants of diabetic mothers may have increased adiposity resulting from storage of the fat generated from the conversion of high glucose supply from the hyperglycemic mothers. These fat stores may be visually obvious to the examiner and can also be objectively identified by measuring body composition, using indices such as an elevated weight, mid-upper arm circumference, and/or triceps skinfold thickness in the absence of an elevated length and head circumference. During the early neonatal period, infants of diabetic mothers are important to identify because they are at higher risk for hypoglycemia, hypocalcemia, polycythemia, and congenital malformations.
Anthropometric and Growth Assessment
Growth Charts
Infant growth charts have been developed to provide normative growth references for monitoring an infant’s postnatal growth. Even though this information is not diagnostic, the data do help differentiate infants who are growing well from those who need more support and/or additional follow-up assessments. Several growth charts exist, each of which have strengths and limitations ( Table 22.1 ). Broadly, these growth charts show normative data of serial anthropometric measurements of fetuses at the time of their birth (intrauterine chart) or preterm infants (postnatal charts). Because the ideal preterm infant growth is not yet defined, many expert groups recommend that the growth of preterm infants should mimic that of the fetus, as seen in intrauterine growth charts. Fetal growth slows down in late pregnancy for two reasons—the space limitations of the in-utero environment and slowing from the peak growth at about 34 weeks, which is displayed on intrauterine growth charts as a flattening after about 36 weeks. Preterm infants do not always slow their growth as much as their fetal counterparts at the same gestational age and can demonstrate some catch-up growth at this age. ,
| Growth Curve | Intrauterine Growth or Postnatal Growth | Population (Including #) | Strengths | Limitations | Access |
|---|---|---|---|---|---|
| Fenton 2013 | Intrauterine combined with term postnatal growth | Represents 3,986,456 preterm births from developed countries (Germany, United States, Italy, Australia, Scotland, Canada) | Harmonized with the WHO Growth Standard Produced using meta-analysis; age is actual age instead of completed weeks; largest sample size Use for classifying infants <36 weeks: SGA/AGA/LGA status, for older gestational age use the 6-country meta-analysis | Should not be used to assign size for gestational age at birth after 37 weeks; use the WHO Growth Standard for these infants; the 6-country meta-analysis can be used for gestational ages 22–42 weeks | PDFs, z -score/percentile calculators, apps, and growth chart curve data for noncommercial use available at www.ucalgary.ca/fenton/ |
| Olsen 2010 | Intrauterine | Represents 257,855 preterm births from 33 US states who survived to discharge | Useful for assessment of size for gestational age at birth for American infants May be used for classifying infants <36 weeks: SGA/AGA/LGA status | Limited use as a growth chart after 36 weeks because fetal growth slows in late pregnancy | https://downloads.aap.org/DOSP/2020OlsenCurveUpdated.pdf |
| INTERGROWTH -21st | Postnatal growth | Represents 224 singleton preterm births (mean gestational age, 35.5 weeks) from antenatally enrolled mothers of low-risk birth with early prenatal care in 8 worldwide locations (Brazil, Italy, Oman, United Kingdom, United States, China, India, Kenya) | More specific for EUGR Harmonizes with WHO | Small sample size at low gestational ages; not data informed <33 weeks Overestimates SGA because growth-restricted infants were excluded | PDFs and apps available at https://intergrowth21.tghn.org |
| WHO 2006 | Postnatal growth for term infants and for preterm infants after term corrected age | 8,440 healthy breastfed infants from diverse ethnic backgrounds | Selected children from communities in which economics were not likely to interfere with growth Longitudinal measures to 2 years of age; z-score charts also available Use for growth monitoring of term infants and preterm infants after term postmenstrual age (using age adjusted for prematurity) Use for classifying infants GA 37 weeks+: SGA/AGA/LGA status | It is appropriate to use these charts to assign size for gestational age at birth for infants 37+ weeks | https://www.who.int/childgrowth/standards/en/ |
Most infants usually lose some weight after birth due to changes in total body water and in nutrient supply, which transfers their growth chart position to lower percentiles. Hence, at any age after the day of birth, weight less than the 10th percentile should not be used to define size for gestational age and categorize infant growth.
Growth Velocity
A goal weight gain of 15 to 20 grams/kilogram/day (g/kg/day) after the postnatal weight loss and up to 36 weeks’ postmenstrual age ( Fig. 22.2 ) is required to maintain intrauterine growth rates, as recommended to support optimal neurodevelopmental outcomes for premature infants.
For the growth of head circumference, estimates from fetal and postnatal growth rates show an increase of approximately 1 to 1.3 centimeters per week (cm/week) until 30 weeks, which then decreases to about 0.4 cm/week by 40 weeks (see Fig. 22.2 ). For growth measured in length, estimates from fetal and postnatal growth rates are a velocity of approximately 1.2 to 1.4 cm/week to 34 weeks, decreasing to about 0.8 cm/week by 40 weeks.
After 36 weeks’ corrected age, the expected weight gain slows to less than 15 g/kg/day. It can be useful to change at this point to grams per day and aim for 20 to 40 g/day until 2 months’ corrected age. Using a growth chart to examine the patterns of weight, head, and length growth from birth adds to the perspective of understanding the growth history, which assists in assessing an infant’s weight in relation to their length and head size and in setting expectations for the future. The typical growth pattern of preterm infants is to lose weight in the first days of life, which places them lower on growth chart percentiles. Not all infants lose weight after birth, and losing no weight may occur more frequently among SGA infants. Head growth seems to be the priority for growth in the first weeks and months of life to gradually catch up to the population average as represented by the intrauterine and Fenton growth charts’ median. Weight catch up generally occurs after term age.
The goal for infants to grow at intrauterine rates applies well to weight gain, because many infants gain weight approximately parallel to the curves on growth charts. Length growth seems to have a lower priority for preterm infants in their first months, because length gain is usually slower than intrauterine rates until after term age. It is important to consider parental size, because parents with short stature are not likely to have the tallest children. Small size at birth may also be due at least in part to socioeconomic disadvantages and thus may identify parents who need additional support.
If preterm infants are plotted on growth charts without correcting for their preterm birth, their size will appear inappropriately small on a growth chart. For example, a 25-weeks’ gestational age female infant with a birth weight of 700 g is at the median on a Fenton preterm growth chart, which reveals this as a normal weight. If this same infant was inappropriately plotted at birth on the World Health Organization (WHO) Growth Standard as a term infant, she appears to have an extremely low birth weight. In the case of a preterm, VLBW infant, it is necessary to continue to adjust for an infant’s early birth throughout the first 3 years of life to accurately assess size. Therefore it is very important to correct for prematurity when an infant is transitioned to growth charts that begin at term/40 weeks’ gestational age, such as the WHO Growth Standard.
The American Academy of Pediatrics recommends that the phrase “corrected gestational age” be used until up to 3 years of chronologic age for infants who are born prematurely. It recommends that corrected gestational age be “calculated by subtracting the number of weeks born before 40 weeks of gestation from the chronological age. Therefore, a 24-month-old, former 28-week gestational age infant has a corrected age of 21 months according to the following equation: 24 months − [(40 weeks − 28 weeks) × 1 month/4 weeks].”
In the NICU, it is important to assess growth patterns from birth by examining the patterns of weight, length, and head circumference together. Short-term assessments have high variability due to fluid fluctuations. Caution should be used when interpreting weight gain of an infant with fluid accumulation, such as infants with lymphatic malformations, hydrops fetalis, cardiac lesions, renal anomalies, or hydrocephalus, because weight will be elevated by the excess fluid. Likewise, after diuretics are started, an infant is likely to temporarily fail to meet weight gain goals due to fluid weight loss. Irremovable equipment such as respiratory support and drainage tubes may hinder precise measurements. Monitoring weight gain over a 5- to 7-day period instead of daily weight gain improves the understanding of growth velocity.
To calculate growth velocity, the Average 2-point method is the most accurate calculation (see Fig. 22.3 for an example) because the Early 1-point method overestimates weight gain, especially for time periods greater than 1 week. Although growth charts reflect cross-sectional fetal growth, many preterm infants achieve similar growth rates after the postnatal weight loss phase. ,
The best practice is to monitor length and head circumference weekly and to use this information along with weight changes to make anthropometric assessments. Recumbent length is best measured with use of a solid length board and two examiners. One person holds the infant’s head against the fixed headboard while the other person gently extends the infant’s legs and places the footboard at the flat feet. For an infant who is too unstable for this measurement, a nonstretchable tape measure can provide a length estimate, , but it is better to use a length board whenever possible. Head circumference (occipitofrontal circumference) is measured with a nonstretchable tape measure around the widest part of the infant’s head. The tape measure will cross the infant’s skull over the frontal bones, along the parietal bones, and over the occipital bones. The head circumference should be measured at delivery but also after 24 hours of life or later in infants who experienced labor, as the skull shape will normalize and soft-tissue swelling and molding will resolve. Length and head circumference should be measured to the nearest one decimal place (e.g., 25.3 cm) and plotted weekly through hospitalization.
Using Z -Scores to Describe Growth Rates
Z -scores define the size of an infant compared with a growth chart at a specific age, in terms of how many standard deviations the infant’s measure is above (positive values) or below (negative values) the median of the growth chart ( Fig. 22.4 ). By examining two or more z -scores for an infant’s anthropometric measurements, the changes in the z -scores reflect whether the infant is maintaining the growth velocity reflected in the growth chart over that time period. As with any anthropometric measurements, z -score fluctuations are normal and expected. Do not expect an infant to maintain z -scores identical to previous measures. As with weight gain monitoring, z -score changes should be assessed over at least 5- to 7-day periods instead of shorter time periods.
If an infant is not maintaining their weight, length, and/or head circumference z -scores after the postnatal weight loss phase in their first week of life, they may be finding their genetic potential, or their growth may be limited by insufficient nutrition or morbidities. When an infant is not maintaining their z -score(s), ensure that nutrition is adequate and not limiting. See Table 22.2 . Many genetic and environmental factors contribute to an infant’s size, which can be indicated at least in part by parental size. The birth z -score is not an appropriate weight gain goal, as the postnatal weight loss that most babies experience after birth lowers the magnitude of their z -score goals. Instead, use the z -score at day 7 as the starting value for comparisons, but do not expect that z -scores will be perfectly maintained, because growth fluctuations are normal.
| Problem | Solution |
|---|---|
| Inadequate intakes due to inadequate nutrition provision , or intolerance(s) | Refer to dietitian Optimize nutrient intakes |
| Human milk has variable composition | Refer to dietitian Assess milk for true protein and energy content Increase fortification of inadequate nutrients |
| Previous fluid overload | Examine growth patterns from birth, consider past and present fluid status |
| Excess weight at birth due to poorly controlled maternal diabetes or excess fluid (e.g., hydrops) | Reassess expectations; when infants have excess weight gain from poorly controlled diabetes, they are likely to have slower subsequent weight gain velocity as they follow their genetic potential for growth |
| Time period of assessment is too short, which can under- or overestimate growth | Assess growth by looking at the patterns of weight, length, and head growth on a growth chart; avoid calculating weight gain velocity over periods shorter than 5–7 days |
Precise weight gain goals can be found by using the website https://www.PediTools.org , but caution is needed when applying these precise numerical goals because individual patient growth is somewhat variable day to day and week to week. Whether estimating precise weight gain goals is superior to examining growth patterns on a growth chart has not been validated. One concern with using these precise weight gain goals is that infants are not likely to achieve any specific numerical growth velocity goals, so using numerical z -score values could lead to excess concern.
Body Mass Index
Body mass index (BMI) is a common measurement for older pediatric and adult populations, and there is increasing interest in using it in infancy ( Fig. 22.5 ). One large cohort study found that infants 4 to 6 months of age with a BMI > the 85th percentile had a higher risk of developing childhood obesity. However, these findings could be due to a few study patients with extreme BMIs, because sensitivity for identifying at-risk infants was only 33%. This study did not account for gestational age or history of prematurity.
In older age groups, such as adults, BMI is primarily used to identify high body fat in older populations. A detailed magnetic resonance imaging study in average preterm infants of former 28 weeks’ gestational age found that both BMI and weight at hospital discharge were strongly associated with total body fat ( r , 0.95 and 0.89, respectively), whereas waist circumference was not associated with body fat ( r , 0.28).
BMI, and other similar indexes such as the ponderal index, do not discriminate between fat and lean tissue. The use of these indices in preterm infants could incorrectly assume that an infant has high body fat if her lean body mass is higher than average or when length growth lags behind weight gain. For this reason, preterm infant nutrition should not be restricted if a preterm infant’s BMI is elevated. Care is needed in the first years of life because any error in age will provide the incorrect BMI reference interval. BMI should not be used with stunted infants or those with restricted length, because these infants are not represented in the reference intervals.
Body Composition Modalities
Direct measurements can be limited by resource availability and technical challenges. Table 22.3 summarizes the strengths and limitations of each of these methods. Publication of normative body composition growth charts allows for comparison of preterm and term infants’ measures. Caution is needed when using body composition measurements in care, because validated techniques to alter care based on these measurements have not been developed. Nutrition should not be provided outside of recommended intake levels (above or below the recommendations) in the neonatal intensive care unit (NICU) even if body composition measures exceed or are below reference intervals. It is not yet known if low lean mass of growing preterm infants can be improved with dietary changes, especially those with morbidities, SGA, or previous nutritional intolerance.
| Method | Parameters | Strengths | Limitations |
|---|---|---|---|
| Air displacement plethysmography | Body fat percentage, fat mass, fat free mass | Quick test Designed for longitudinal, repeated measurements Easy interpretation Growth charts available Safe, noninvasive | Requires machine Only tests infants breathing room air |
| Magnetic resonance imaging | Adipose tissue volume, fat mass, and fat-free mass | Safe, noninvasive Assesses regional distribution of adipose tissue, not only volume | Expensive Expert interpretation required Patient must be able to go to scanner and must be still Requires technician and machine |
| Isotope (deuterium) dilution , | Total body water, fat-free mass, fat mass | Accurate Noninvasive | Requires mass spectrometry Expert interpretation required Requires technician and machine 2–4-hour completion time Hydration status will affect results |
| Bioelectrical impedance analysis | Fat mass, fat-free mass | Quick test Safe, noninvasive | Poor individual accuracy |
| Skin fold thickness | Total body fat | Safe, noninvasive Quick test Can be measured regardless of clinical status | Poor accuracy, especially for preterm infants who have low subcutaneous fat |
| Dual energy x-ray absorptiometry | Bone mineral density, fat-free mass, fat mass | Accurate for bone mineral density Quick test | Patient must be able to go to scanner and must be still Radiation exposure Inaccurate fat measure Requires machine and technician |
| Mid-upper arm circumference | Quick test Safe, noninvasive | Limited evidence does not support its use for preterm infants , Does not discriminate between fat and lean tissue |
When direct measurement is unavailable, there are validated equations to predict body fat in infants over 2000 g. ,
Neonatal fat mass (kg) = (Body weight [kg] × 0.39055)+ (Flank skin fold [mm] × 0.0453) − (Length [cm]× 0.03237) + 0.54657
Neonatal body fat percentage = Fat mass (kg)/Body weight (kg) × 100
Evidence of Healthy Body Composition
Body composition measurement provides additional information that may assist the nutritional assessment over anthropometric measurement alone; however, validated methods to use this information have not been established. Body fat provides insulation to aid in temperature stability and is an important alternative substrate for the glucose-dependent brain while early oral feeding is established. In a study of term infants, low fat mass and low body fat percentage had greater predictive value than low birth weight percentile for neonatal morbidities including hypothermia, poor feeding, and extended length of stay, suggesting that body composition data may assist differentiating infants with fetal growth restriction from small, healthy infants better than weight percentiles do. Body composition may provide more information to aid assessments and may reflect the infant’s medical and nutritional history rather than provide guidance to change care.
The body composition of former preterm infants at term-corrected gestational age is different than the body composition of equivalent term counterparts. Former preterm infants have less lean tissue but similar fat mass at term-corrected gestational age, resulting in a different proportionality, including a higher proportion of body fat. Researchers have expressed great concern that this higher proportion of body fat is a concern for later body composition, but it should be noted that term infants also increase their proportion of body fat in the first months of life ( Figs. 22.6 and 22.7 ). Lean body mass growth is an indication of protein accretion as well as organ and brain growth. Higher lean body mass gain among preterm infants during NICU hospitalization is associated with higher neuronal processing speed and superior neurodevelopment. , Postnatal fat accretion, which occurs at an earlier postmenstrual age in preterm infants (i.e., before 40 weeks’ postmenstrual age) compared with term infants (i.e., after 40 weeks’ postmenstrual age), has been suggested to be a normal postnatal development for all infants.
