Serum growth-promoting activity in human newborns. Relationship of thymidine activity with birth weight and the length of gestation.

Thymidine Activity (TA) was measured by the effect of serum upon incorporation of 3H-thymidine into human lectin-activated lymphocytes in 54 newborns: 32 full-term, 11 pre-term and 11 with intra-uterine growth retardation (IGR). Capillary blood was collected at 0-6 hours, with routine samplings. TA values were lower in preterm (0.476 +/- 0.079 U/ml) than in IGR (0.910 +/- 0.118 U/ml, p less than 0.01) and in full-term neonates (1.237 +/- 0.60, p less than 0.001), and also in IGR newborns than in newborns of normal weight (p less than 0.025). In preterm neonates TA was significantly correlated with gestational age (r = 0.715, p less than 0.02), but no such correlation existed in IRG and full-term neonates. When TA values were plotted against birth weight, a correlation was found in preterm (r = 0.715, p less than 0.02) and in IGR newborns (r = 0.714, p less than 0.02), but not in full-term neonates. Longitudinal study up to 21 days did not show significant changes in full-term newborns, while in preterm and IGR neonates TA increased progressively to reach normal values at the 21st day. The correlations observed in newborns between TA, birth weight and gestational age, and the postnatal normalization in newborns with low birth weight, show that TA directly reflects the nutritional state of the fetus.