Key topics

The infant who is small for gestational age
Classification of small for gestational age infants
Causes of intrauterine growth restriction
Problems to be expected in the growth-restricted fetus and small for gestational age infant
Management of the low birthweight infant

Introduction

Newborn infants can be classified according to birthweight, gestational age or size for gestational age (Table 12.1). Low birthweight is defined as less than 2500 g. However, a low birthweight (LBW) infant may be that size because they are either preterm or small for gestational age (SGA), or both. The problems these infants develop tend to be determined by whether the infants have been born too early or are born too small for the duration of gestation. The problems associated with LBW are described here. Those predominantly associated with prematurity are described in Chapter 11.

Table 12.1 Classification of newborn infants according to birthweight, gestational age or size for gestational age

Factor	Terminology	Incidence (%)
Birthweight
<2500 g	Low birthweight (LBW)	6.5
<1500 g	Very low birthweight (VLBW)	1.3
<1000 g	Extremely low birthweight (ELBW)	0.6
Gestational age (completed weeks after last normal menstrual period)
<37 weeks	Preterm	8.4
>41 weeks	Post-term	0.6
Size for gestational age
Weight between 90th and 10th centiles for gestation	Appropriate for gestational age (AGA)	80
Weight <10th centile for gestation	Small for gestational age (SGA)	≈10
Weight >90th centile for gestation	Large for gestational age (LGA)	≈10

The Infant Who Is Small for Gestational Age

Although several synonyms have been used to describe the growth-restricted infant, including dysmaturity, light for dates, small for dates and intrauterine growth restriction, the preferred term is now SGA. There has been no such uniformity in the definition of SGA. From a statistical viewpoint, infants weighing more than two standard deviations below the mean can be defined as SGA, regardless of gestation. However, because they share common clinical problems, infants below the 10th centile for weight are often regarded as ‘SGA’. It must be recognized that some of these will in fact be appropriately grown and are just smaller than average, for familial or racial reasons. Furthermore, many neonates with birthweight above the 10th centile will demonstrate evidence of acute or chronic weight loss, and should therefore fall into the spectrum of the growth-restricted infant. Variables such as sex, race and altitude should be considered when determining growth curves, because a birthweight below the 10th centile in one population may not fall below the 10th centile in another, even though both may be growth restricted. Table 12.2 lists important factors that predict birthweight of an individual fetus or neonate for a given population.

Table 12.2 Determinants of birthweight

Maternal factors before conception	Stature
	Weight
	Genotype
	Race
	Age
	Parity
	Socioeconomic status (occupation, education, income)
Factors at or around conception	Fetal genotype
	Singleton or multiple conception
	Fetal sex
	Genetic anomaly (chromosomal or major gene locus)
Factors between conception and birth	Altitude above sea level
	Fetal or maternal infection (rubella, malaria)
	Maternal work and ability to rest
	Maternal cigarette smoking
	Maternal diet
	Maternal alcohol, drugs, medications
	Placental dysfunction (e.g. PET)

CLINICAL TIP: It is important that weight and length measurements are plotted on an appropriate growth chart. WHO have issued growth charts for preterm and term babies based on a breastfed population, which mean breastfed infants will no longer appear to be dropping centiles. Down’s syndrome specific charts also exist. Growth-restricted infants should be plotted on a normal growth chart appropriate to their gestational age.

Link to Nursing the Neonate: The small baby. Chapter 5, pp. 65–70.

Classification of Small for Gestational Age Infants

Subdividing the heterogenous SGA population into subgroups offers the potential to better understand the underlying cause, and therefore allows a more accurate prognosis and appropriate postnatal management:

SGA but appropriately grown (constitutionally small)
SGA due to growth restriction (IUGR).

It is important to attempt to define the neonate ‘starved’ as a result of IUGR. These babies show greatest growth failure in terms of weight, then length; head circumference is the least affected. There is little subcutaneous fat, the skin may be loose and thin, muscle mass is decreased, especially the buttocks and thighs, and the infant often has a wide-eyed, anxious look. This pattern can be distinguished from the uniformly growth-restricted type, which implies either a fetal cause (e.g. chromosomal) or a very early insult.

Subcutaneous fat can be objectively assessed by measuring the circumference of the mid-upper arm or by measuring skinfold thickness with special calipers.

Causes of Intrauterine Growth Restriction

The causes of IUGR can be classified according to whether they are fetal, placental or maternal (Table 12.4). The growth failure can be classified as ‘intrinsic’, which implies an abnormality at the time of conception or within the first trimester (fetal and some maternal causes), or ‘extrinsic’, implying a later onset of growth restriction (placental or maternal causes). See Table 12.3.

Table 12.3 Classification of the growth-restricted fetus and the SGA infant. Many growth-restricted infants exhibit a mixture of classifications

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