Low birth weight - defined as newborns
weighing less than 2 500 grams - is an important indicator of infant health because of the
close relationship between birth weight and infant morbidity and mortality. There are two
categories of low birth weight babies: those occurring as a result of restricted foetal
growth and those resulting from pre-term birth. Low birth weight infants have a greater
risk of poor health or death, require a longer period of hospitalisation after birth, and
are more likely to develop significant disabilities (UNICEF and WHO, 2004).
Risk factors for low birth weight
include adolescent motherhood, a previous history of low weight births, engaging in
harmful behaviours such as smoking and excessive alcohol consumption, having poor
nutrition, a low body mass index, a background of low parental socio-economic status, and
having had in-vitro fertilisation treatment.
One-in-fifteen babies born in OECD
countries in 2009 - or 6.7% of all births - weighed less than 2 500 grams at birth (Figure 1.8.1). The Nordic countries (Iceland, Sweden and Finland), Estonia,
Ireland and Korea reported the smallest proportions of low-weight births, with less than
5% of live births defined as low birth weight. Alongside a number of emerging countries
(India, South Africa and Indonesia), Turkey and Japan are at the other end of the scale,
with rates of low birth weight infants above 9% (Figure 1.8.1).
Since 1980, and more so after 1995,
the prevalence of low birth weight infants has increased in most OECD countries (Figure 1.8.2). There are several reasons for this rise. The number of
multiple births, with the increased risks of pre-term births and low birth weight has
risen steadily, partly as a result of the rise in fertility treatments. Other factors
which may have influenced the rise in low birth weight are older age at childbearing, and
increases in the use of delivery management techniques such as induction of labour and
caesarean delivery, which have increased the survival rates of low birthweight babies.
Japan, Portugal and Spain have seen
large increases in the past three decades, such that the proportion of low birth weight
babies in these countries is now above the OECD average (Figure 1.8.2). This contrasts with the proportions of low birth weight babies
in Chile, Poland and Hungary which have declined over the same time period. Little change
has occurred in Finland, Sweden and Denmark, although Iceland and Norway saw rises.
Figure 1.8.3 shows some correlation between the percentage of low birth
weight infants and infant mortality rates, a relationship which is stronger with the
inclusion of emerging countries. In general, countries reporting a low proportion of low
birth weight infants also report relatively low infant mortality rates. This is the case,
for instance, in the Nordic countries. Japan is an exception, since it reports the highest
proportion of low birth weight infants but one of the lowest infant mortality rates.
Comparisons of different population
groups within countries show that the proportion of low birth weight infants can also be
influenced by differences in education, income and associated living conditions. In the
United States, marked differences between groups in the proportion of low birth weight
infants have been observed, with black infants having a rate almost double that of white
infants (NCHS, 2011). Similar differences have also been observed among the indigenous and
non-indigenous populations in Australia, Mexico and New Zealand, often reflecting the
disadvantaged living conditions of many of these mothers.
Definition and comparability
Low birth weight is defined by the
World Health Organization (WHO) as the weight of an infant at birth of less than 2 500
grams (5.5 pounds) irrespective of the gestational age of the infant. This is based on
epidemiological observations regarding the increased risk of death to the infant and
serves for international comparative health statistics. The number of low weight
births is then expressed as a percentage of total live births.
The majority of the data comes
from birth registers, however for Mexico the source is a national health interview
survey. A small number of countries supply data for selected regions or hospital