In this cross-sectional study of 178 pregnant women between the 7th and 42nd week of pregnancy, we analyzed correlations between erythropoietin (EPO) and vitamin B12 (B12) in different stages of pregnancy and in relation to hemoglobin (Hb) levels. Patients with hypertension, fetal growth retardation and severe systemic diseases were excluded. EPO (by ELISA), B12 (by RIA) and Hb were assayed in the same blood sample taken on admission. On the basis of weeks of pregnancy, EPO levels and B12 levels, the 178 subjects were found to fall into two clusters, before and after the 27th week of gestation. The correlation coefficient between EPO and B12 was highly significant in the first group but not in the second (R = ––0.33; p < 0.01). When the patients were divided on the basis of Hb levels ( 11 g/dl (R = ––0.44; p < 0.001) and not in the 72 anemic subjects. Moreover, in the former group the correlation between EPO and B12 was high before and after the 27th week, unlike in the latter group for which no significant correlation was found. These results suggest that EPO and B12 act together to establish normal erythropoiesis in pr