Thymidine factor in human cord sera and relationship to birth size, gestational age and insulin

Thymidine factor was studied in human cord sera from 44 neonates with gestational ages ranging between 36 and 43 weeks. Thymidine factor was determined by the uptake of 6-3H-thymidine using isolated chick embryo cartilage cells. The thymidine factors increased with gestational age and those of neonates at 40 weeks of gestation were 0.89 +/- 0.17 U/ml (mean +/- SD), but were significantly lower after 42 weeks of gestation. There was no correlation between thymidine factor in cord sera and birth weight or size. There was a positive correlation (p less than 0.02) between insulin level and thymidine factor in cord sera. Our data suggest that somatomedin measured as thymidine factor might not have a major role in fetal growth and insulin might be an important factor in the synthesis of somatomedin by the fetus.