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.