Discussion

Different reasons can lead to an interest in identifying IUGR babies. Bakketeig's perspective is that of a clinician in an industrialized country, primarily interested in individual case management. From that point of view it is important to diagnose IUGR early, before the infant is born, in the hope of preventing a portion of fetal deaths that are associated with growth retardation by delivering such babies early. Although it has not been possible to confirm this in randomized trials, obstetricians like Goldenberg, for instance, are of the opinion that much of the reduction in fetal deaths in the US over the last 30 years results from these efforts to identify babies undergoing IUGR and delivering them early. If the purpose is to identify populations at risk as targets for intervention, it is possible to look for growth retardation not in utero, but at birth. The desirable specificity of reference values and growth curves depends to a large extent on this distinction and on the use for which they are intended.

If the intended purpose is individual case management, adjustments of growth curves for parity of the mother and sex of the infant seem justified since firstborns and girls are on average smaller and lighter, without this having any effect on outcome. For the reasons presented by Bakketeig, adjustments for prior birth outcomes (taking into consideration the smaller size of firstborns), maternal birthweight and adult height also appear desirable, it these data are available.

Based on the observation that children of socio-economically advantaged classes in developing countries follow growth reference curves of healthy, well-nourished children in developed countries, and that children of the same genetic background show widely differing growth performance depending on the environment in which they grow up, the prevailing opinion today is that people of all races have the same growth potential, even though this growth potential may not be attained in one generation, and that country- or race-specific growth references are therefore not appropriate. Making adjustments for the height of stunted parents, for instance in South Asia, could reinforce the wrong impression that children of this region are born small for genetic reasons and that not much can be done about this. Growth curves should certainly not be adjusted for factors that may be a cause of growth retardation.

Even though they are all interested in identifying children who are not reaching their growth potential, different groups of professionals approach the problem differently and use different classification schemes. Obstetricians responsible for individual case management and epidemiologists interested in fetal growth describe fetuses and infants as SGA (usually with a cut-off at the 10th centile) or IUGR and advocate the use of ultrasound for diagnosis and documentation of growth retardation. Few of them look at body proportions, and those who do usually use ponderal index as an indicator and for categorization of newborns as proportionate or disproportionate. Pediatricians and nutritionists, who are primarily interested in postnatal growth, monitor the infants' weight and height and use primarily weight-for-age, height-for-age and weight-for-height as indicators. The most frequently used cut-off to trigger intervention is weight-for-age below - 2SD of the NCHS reference population. Low height-for-age and weight-for-height are used to classify children as stunted and wasted respectively. These indicators are primarily intended to guide intervention. Since pre- and postnatal growth are one continuous process, a harmonization of approaches and classification schemes would be desirable.

Clinicians and other practitioners use cut-off points and thereby establish binary divides between those considered growth retarded, stunted, wasted, etc. and those who are not, those who are in need of an intervention and those who are not. More epidemiologically interested scientists concede that cut-offs are useful for triggering intervention, but emphasize that growth is a continuous process and so are the risks associated with variations in growth. In developing countries, for instance, where the whole distribution curve for growth is shifted to the left by one or even two SD in comparison to reference curves from developed countries, the magnitude of the public health problem may be underestimated by only looking at the proportion of those falling below a cut-off point.

It is not yet certain whether infants of different races, born at a particular weight-for-gestational-age, are all at the same or at different risks for health outcomes. It seems plausible that not only the degree of growth retardation, but also its etiology is an important determinant of risk of various undesirable outcomes; this, however has not yet been well documented. Smoking mothers give, on average, birth to smaller babies, but a comparison of offspring of mothers who smoked during all pregnancies and women who smoked only during some did not show much difference in terms of morbidity and mortality outcome.

It is difficult to estimate the portion of IUGR that is genetically and the portion that is environmentally determined. Comparisons of siblings with half-siblings in Scandinavia should be able to clarify the situation in developed countries to some extent. The current consensus is that genetic influences are relatively unimportant, accounting for only 10 to 15% of the variation in birthweight, and that in developed countries they have a greater influence than in developing countries in which any genetic effects are obscured by much larger environmental influences. A partial reflection and illustration of this is that correlations between maternal weight gain in pregnancy and birthweight tend to be low and not statistically significant in industrialized countries, whereas they can be significant in developing countries where most mothers have low body mass at conception.