Serum growth-promoting activity in normal and hypotrophic fetuses at midpregnancy

Blood from 24 human fetuses aged 19-24 wk was collected by ultrasound-guided puncture of the umbilical cord in utero, performed for prenatal diagnosis of mother to fetus transmissible infections. Fetal serum growth-promoting activity (thymidine activity) was measured by its effect on 3H-thymidine incorporation into human lectin-activated lymphocytes. Ten blood samples were obtained at 19-22 wk of pregnancy and 14 at 23-24 wk. The pregnancies were maintained and the fetuses delivered, free of infection, at 38-40 wk, nine of them being small for date and 15 having a normal weight for gestation age. The bioassayable thymidine activity was significantly lower in the hypotrophic (0.84 +/- 0.04 U/ml) than in the normal fetuses (1.28 +/- 0.09 U/ml) whatever the time of sampling. Thymidine activity was significantly negatively correlated with gestational age in the normal for date fetuses, not in the small for date. It is suggested that early measurement of thymidine activity in fetal blood might be of value in the assessment of fetal growth despite the fact that the tissue growth factors may be more important in fetus than are the serum factors.