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close this bookCauses and Mechanisms of Linear Growth Retardation (International Dietary Energy Consultative Group - IDECG, 1993, 216 pages)
close this folderPrenatal influences on postnatal growth: Overview and pointers for needed research
View the document(introductory text...)
View the document1. Introduction and background
View the document2. Fetal growth
View the document3. Intrauterine growth retardation
View the document4. Small-for-gestational-age infants
View the document5. Genetic and environmental factors
View the document6. Reference values for fetal growth
View the documentReferences
View the documentDiscussion
View the documentReferences

5. Genetic and environmental factors

The Louisville Longitudinal Study of Twins (Falkner & Matheny, 1993) provides evidence of a strong positive correlation between placental weight and birth weight. In monozygous twins, who are phenotypically similar, differences in birth weight and in postnatal growth may be determined by placental weight. It was found that the concentrations of several biochemical and nutritional substances were the same in the placental segments belonging to each twin, but because of the difference in placental weights the absolute amounts were different. Could it be that there is a critical placental mass, below which postnatal growth deficit occurs?

Fig. 1. Six monthly increments in length and height for Twins A & B.

A pair of male monozygous twins from the Louisville Study illustrates many features relevant to this discussion. The smaller twin's (Twin A) birth weight was 52% of that of his larger brother (Twin B). Twin A's placental segment represented only 46% in mass of Twin B's. Most importantly, with both twins born at 39 weeks' gestation, Twin A was a SGA infant.

Fig. 1 shows that Twin A exhibited marked catch-up growth in length during the first nine months to one year in an attempt (hypothetical) to compensate his length deficit of 7.0 cm at birth. Thereafter, both twins grew at approximately the same velocity. However, Twin A never exhibited sufficient catch-up for a long enough period to achieve his co-twin's stature. Both twins started their adolescent growth spurts at the same age (16 years). The within-pair difference was then 8.0 kg in weight and 5.4 cm in stature.

Because head circumference is a good indicator of brain mass (Brandt, 1986; Dobbing & Sands, 1978), it needs to be taken into account in any discussion of IUGR. Placental factors should also always be considered.