Regulation of erythropoiesis in fetus and mother during normal pregnancy

The activity of the hormone erythropoietin was assayed (1) in the amniotic fluid of healthy women in the tenth to 16th weeks of pregnancy who had artificial interruption of pregnancy and (2) in the urinary output of healthy pregnant and nonpregnant women. Erythropoietin activity in concentrated urine increased significantly between the 24th and 31st weeks of pregnancy. No erythropoietin activity was demonstrated in amniotic fluid at the tenth week of pregnancy, even when the fluid was concentrated, but it was demonstrated in concentrated amniotic fluid from the 11th to 16th weeks. The assay results prove, indirectly, the hypothesis that increased production of erythrocytes in pregnancy is mediated by the erythropoietin humoral mechanism in both fetus and mother. Erythropoiesis in the fetus probably is regulated by erythropoietin as early as the 11th week of pregnancy.