DEFINITION OF LOW BIRTHWEIGHT

A low-birthweight (LBW) baby is arbitrarily defined as weighing less than 2500 g at birth. There are further subdivisions of weight into very low birthweight (VLBW) – weighing <1500 g and extremely low birthweight (ELBW) weighing <1000 g at birth.

Low birthweight may occur because the infant is premature, less than 37 weeks’ gestation, or because the infant’s intra-uterine growth has been restricted.

Intra-uterine growth restriction (IUGR) is defined as a birthweight of less than the 10th centile of the weights for babies at that gestational age. Similarly babies whose birthweight is greater than the 90th centile for weights for babies at that gestational age are defined as large for gestational age (LGA).

It follows that since at any given gestational age 10% of babies are SGA and 10% are LGA then only 80% are of a birthweight that is appropriate for gestational age (AGA).

Although the rate of prematurity varies between communities, a common rate in developed countries is 7%, (and that of post-maturity up to 1%). The relation between growth and gestation is illustrated in Figure 27.1, including approximate percentages of the various combinations of gestation and growth relative to the normal parameters. Note that by these definitions less than 75% of babies are born of ‘normal’ gestation and weight. Note also that some LBW infants may be both premature and IUGR.

Fig. 27.1 Birthweight distribution versus gestation period. Dotted lines show the 10th and 90th centiles.

The growth chart illustrated in Figure 27.1 is from an Australian community. It is not necessarily applicable to all communities because of different genetic growth potentials; for instance the birthweights in a population of some Pacific Island groups would show higher growth centiles and some Indian populations would show lower centiles.

CAUSES OF LOW BIRTHWEIGHT

As stated above, low birthweight may occur because the infant is premature – less than 37 weeks’ gestation – or because the infant’s intrauterine growth has been restricted.

Causes and prevention of prematurity are discussed in Chapter 19, with a summary of causes given in Table 19.1.

The intra-uterine growth of the fetus depends on its inherited growth potential and the effectiveness of the support to its growth provided by the uteroplacental environment. The latter is affected by the presence or absence of maternal disease. The causes of IUGR can be classified according to maternal, placental and fetal causes, and are outlined in Box 27.1. In many cases no precise factor has been identified.

Data are available from several developed countries which show the proportion of babies who are of low birthweight, their perinatal mortality, and the survival of those babies born alive. These data obscure the fact that low-birthweight babies comprise two populations: preterm (premature) babies and small-for-gestational age (SGA) babies.

PROBLEMS OF PREMATURE AND GROWTH-RESTRICTED INFANTS

Distinction needs to be made between the problems of the two groups of infants. In premature infants, but not necessarily in IUGR infants, all organ systems are immature, giving rise to a variety of potential problems, including:

VLBW and ELBW infants are also at risk of persistent patency of the ductus arteriosus, intraventricular haemorrhage (IVH) and its complications, retinopathy of prematurity (ROP), necrotizing enterocolitis (NEC), bronchopulmonary dysplasia (BPD), anaemia and electrolyte disturbances due to renal immaturity.

IUGR infants have a different set of problems, but if they are also premature they also share the problems listed above. Specific problems of the IUGR infant include:

MANAGEMENT OF PREMATURE AND GROWTH-RESTRICTED INFANTS

The care of these infants should be conducted by a paediatrician with the resources available in a neonatal special care nursery (SCN – often called a special care baby unit – SCBU). Most VLBW infants and all ELBW require the care of a specialist neonatal multidisciplinary team in a neonatal intensive care unit (NICU). As mentioned in Chapter 19, a woman who goes into labour before the 36th week of pregnancy should, if possible, be transferred to a hospital with an SCN. If delivery is required before the completion of 32 weeks (in some regions this may be a higher gestation) all attempts should be made to transfer the mother for delivery at a hospital with an NICU.

Key points in the management include:

MORTALITY AND MORBIDITY OF LOW-BIRTHWEIGHT INFANTS

The birthweight and gestational age of the infant are the greatest predictors of morbidity and mortality. Those at greatest risk of complications and of long-term morbidity are the VLBW babies (<1500 g), and at even higher risk are the ELBW babies (birthweight <1000 g).

Low-birthweight infants contribute 70% of early neonatal deaths; the smaller and less mature the infant, the less its chance of survival (Table 27.1). The survival rates depend on the place of birth. Few infants of less than 24 weeks’ gestation survive; neonatal survival of live births at 24–25 weeks’ gestation is 45%, 78% at 26–27 weeks’, 90% at 28–31 weeks’, and over 99% at 32 weeks’ and above (Victoria, Australia, 2006).

The morbidity of the surviving infants has decreased in recent years; the highest morbidity is among infants whose gestational age is less than 27 weeks and whose birthweight is 750 g or less. An Australian study of outcome at 2 years of age of 168 surviving infants (a survival rate of 73%) who were born with birthweights less than 1000 g showed that 51% had no disability, 23% a mild disability, 13% a moderate and 14% a severe disability, 11% had cerebral palsy, 2% were blind and 2% were deaf. For those born 20 years earlier the survival rate was 25%; disability rates were mild 33%, moderate 13% and severe 15%, cerebral palsy 14%, blindness 7% and deafness 5% (Table 27.2).