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Most infants who are SGA have low levels of insulin-like growth factor-1 (IGF-1) and high levels of growth hormone (GH). IGF-1 is the main growth factor during the fetal period. The compensatory increase of GH is ineffective since GH receptors are not yet developed and GH cannot yet play the role of an important growth factor that it will normally assume in the course of the first year of extra uterine life. During childhood GH levels and growth are positively associated with blood pressure. Perhaps this points to a mechanism that could explain the association between low birth weight and high blood pressure later in life.
As long as serum protein is adjusted for, and there is no pre-eclampsia, there is a linear relationship between maternal blood pressure and infant birth weight; this relationship holds true even up to a point where the mothers are mildly hypertensive, i.e. a diastolic blood pressure of around 100.
As Prada pointed out, the dietary
intakes of certain nutrients (vitamin B1 B6, D, E, folacin, iron,
zinc, calcium and magnesium) by pregnant women in the US are consistently lower than
recommended daily allowances (RDAs). Recommendations vary from country to country, however
- an indication of uncertainties about requirements. Persons with nutrient intakes that
are lower than RDAs do not necessarily have to suffer from nutrient deficiencies.
Supplementation trials are needed to establish that there is a nutrient deficiency and
that its correction increases birthweight. Clear evidence in that sense is only available
for balanced protein-energy supplementation of undernourished women. The etiologic role of
micronutrients in IUGR remains to be clarified. Unfortunately few supplementation or
fortification trials with various nutrients have been carried out in developing countries
where the prevalence of nutrient deficiencies is highest. Zinc, iron, calcium, magnesium,
folate, vitamin A and vitamin D appear to be promising candidates for trials in
populations in which they are deficient. A complicating factor is that poor populations
could suffer from multiple nutrient deficiencies (e.g. Fe and folate) that might all need
to be corrected to achieve the desired outcome. Availability of nutrients in adequate
amounts is necessary, but may not be sufficient; conditions that ensure their transfer and
utilization also have to be met. Increased energy intake in pregnant smokers, for
instance, does not prevent fetal growth retardation. For certain nutrients (e.g. Fe) the
range between deficiency and excess is relatively narrow. Supplementing nutrients that are
not deficient in a population's diet have been reported to have had adverse effects, as
illustrated by a protein supplementation trial in Harlem that lowered average birth weight
(Rush et al, 1981). Supplementation of one nutrient could also interfere with or
enhance the absorption of another.
Rush D, Stein Z & Susser M
(1981): Diet in pregnancy: A randomized controlled trial of nutritional supplements.
Alan R Liss: New York.