Dynamic changes in the reticulocyte count and EPO levels after birth. (A) Reticulocyte count was evaluated in C57BL/6 mice every day from P1 to P14 and then every week until 12 weeks of age using ADVIA 120. (B) EPO levels in the serum were measured every other day for the first 2 weeks and then every week until 12 weeks of age. n = 4-13 animals per day; data are presented as mean ± SEM. **P < .01, ****P < .0001, indicated postnatal day vs P1 (ANOVA with Tukey’s post hoc test with corrections for multiple comparisons).