Erythropoietin and cord blood haemoglobin in the regulation of human fetal erythropoiesis.

Erythropoietin was estimated by radioimmunoassay in serum from 78 cord blood samples, collected in the second and third trimesters in 72 pregnancies. In 43 samples obtained during or after normal pregnancy (from 19 to 42 weeks gestation) erythropoietin levels increased with gestation. Cord blood haemoglobin also increased with gestation, but the rate of increase was less during the last weeks of pregnancy. Erythropoietin levels were similar in the cord blood of infants of the same gestation, whether born vaginally or by caesarean section. The fetus can respond to severe anaemia or hypoxia with increased erythropoietin levels as early as 24 weeks gestation. Elevated erythropoietin levels were found in two out of eight infants born after labour in which there was 'acute' fetal distress, suggesting the presence of unrecognized chronic fetal hypoxia in these pregnancies.