Serum somatomedin-C and bioassayable growth-promoting activity (thymidine activity) in appropriate and small-for-gestational-age human newborns.

The serum level of radioimmunoassayable somatomedin-C and the bioassayable growth-promoting activity evaluated by the stimulating effect of serum upon thymidine incorporation into activated lymphocytes have been measured in the blood of term human foetuses. Comparison between those with a low birth weight and those with normal birth weight has shown that small-for-gestational-age subjects have lower somatomedin-C (0.31 +/- 0.03 vs 0.52 +/- 0.03) and thymidine activity (1.03 +/- 0.11 vs 1.50 +/- 0.07) (P less than 0.001). A positive correlation between somatomedin and thymidine activity was found. There was no difference in serum transferrin levels between both groups. It is suggested that somatomedin, and probably other growth-promoting factors measured by the thymidine bioassay, play a role in regulation of the foetal growth.