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Types of IUGR
Association of low birth weight with diarrhoea
Association of low birth weight with respiratory infections
Differential effects in stunted vs wasted IUGR infants
Correspondence: Dr Ashworth
Centre for Human Nutrition, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK
This review aims to quantify the risks of mortality and morbidity associated with intrauterine growth retardation (IUGR). Twenty-nine data sets with birth-weight-specific mortalities are examined to determine whether consistent patterns of risk emerge when data from different populations are compared. Measures of mortality risk are also made with birth weight as a dichotomous variable. Twelve data sets are presented.
From the data available, it is estimated that for term infants weighing 2000-2499 g at birth, the risk of neonatal death is 4 times higher than for infants weighing 2500-2999 g, and 10 times higher than for infants weighing 3000-3499 g. The risk of postneonatal death in term infants weighing 2000-2499 g is estimated to be 2 times higher than for infants 2500-2999 g, and 4 times that of infants weighing 3000-3499 g. Estimates of risk for IUGR infants are less consistent than for preterm infants. This could be due to methodological differences, particularly smaller sample sizes in the studies in developing countries, or may reflect real variation in risk. The latter may be associated with the heterogeneity of IUGR across populations, or to varying risks depending, for example, on which infections predominate or infant age at peak prevalence. IUGR is most prevalent in developing countries and the review therefore focuses on morbidity from diarrhoeal and respiratory infections. Data from nine studies are presented. There is an increased risk of diarrhoea in term infants < 2500 g and an increased risk of pneumonia.
The risks of morbidity and mortality
appear to differ depending on whether infants are wasted or stunted at birth. Stunted
infants of low birth weight have higher neonatal mortality than wasted newborns, but this
could be due to inclusion of infants with congenital anomalies who are often stunted.
Wasted infants are more prone than stunted infants to neonatal morbidity. No comparative
postneonatal data were located.
Studies of morbidity in infants of
low birth weight (LBW) have tended to focus either on problems occurring in the immediate
perinatal period, such as asphyxia and hypoglycaemia, or on long-term developmental
outcomes. Most have been conducted in relatively affluent populations, yet over 90% of
global births < 2500 g occur in developing countries. This review will therefore focus
on the impact of LBW on morbidity and mortality in developing countries, and on diarrhoeal
and respiratory infections as these are major causes of illness and death. Where possible,
infants with intrauterine growth retardation (IUGR) will be distinguished, as such infants
are likely to comprise the majority of low weight births in these countries. This
contrasts with the situation in affluent populations where the majority of LBW infants are
preterm. If the risk of mortality and morbidity in IUGR infants can be quantified, then
the impact of interventions to reduce IUGR incidence can be estimated. The approach taken
in this review will be to examine whether consistent patterns of risk emerge when
mortality and morbidity data from several countries are collated.
There has been inconsistency in the past regarding the definition of LBW, but a weight of < 2500 g at birth is now widely accepted. Several of the studies referred to in later sections of the text, however, have defined LBW as £ 2500 g and used birthweight intervals of 1001-1500 g etc. These 1g differences will be ignored and the intervals will be presented as 1000-1499 g etc.
There is no agreement as to the
definition of IUGR but it may be defined as a birth weight > 2SD below the median for
gestational age. Some investigators use the 3rd or 5th centiles, but many use the 10th
centile as the cut-off point which will result in some small but normal newborns being
wrongly designated as growth-retarded. This inconsistency in the cut-off hampers
comparative analysis. A further hindrance is that at least a dozen reference populations
are used, and some differ substantially. For example, the 10th centile birth weights for
boys at 40 weeks gestation are 2630 g and 3030 g respectively for the reference
populations of Lubchenco et al (Denver) and Arbuckle et al (Canada) (World Health
Organization, 1995). The reasons for these differences are well established (World Health
Organization, 1995; Miller, 1981; Rosso, 1989). For all the reference populations, the
10th centile exceeds 2500 g. In this review, the descriptor will be indicated where
possible to facilitate interpretation.
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