Are elevated fetal nucleated red blood cell counts an indirect reflection of enhanced erythropoietin activity?

Objective: Animal and human studies demonstrated elevated erythropoietin (EPO) levels in response to intrauterine hypoxic events. Other studies documented an association between fetal hypoxia and elevated nucleated red blood cell (NRBC) counts and have speculated that it is the elevation of EPO that results in an increase in NRBC counts. Thus, the purpose of our study was to determine the correlation between EPO levels and NRBC counts in the human fetus. Study design: Data were collected prospectively between April and July of 2003. Term singleton pregnancies were eligible to participate in the study. Umbilical cord blood was collected immediately after birth for determination of fetal EPO levels and NRBC counts. Results: Forty pregnancies formed the study population. The mean gestational age at delivery was 39.5 ± 1.2 weeks (± SD) and the mean birth weight was 3500 ± 372 g. The median EPO (mU/mL) was 34 (range 13-427). The median NRBC/100 white blood cells was 10 (range 0-150). A simple regression analysis indicated that NRBC counts are significantly and positively correlated with EPO (P = .0004, R² = 0.287). Conclusion: Our results suggest a significant association between EPO and NRBC counts in term singleton fetuses. These results support the hypothesis that fetal NRBC and EPO are interrelated. However, the relatively low R² indicates that there are other (yet to be determined) hypoxia-derived mediators that result in an elevation of fetal NRBC counts.