Cord blood nRBC distributions in a low-risk population: can they identify the time of fetal injury?

Abstract

The hypothesis of this study is that nucleated red blood cells (nRBCs) released in response to low [O₂] are correlated with components of such a response (e.g., reticulocytes and increased erythrocyte number), and that such a response is correlated with abnormal placental growth, pathology, and reduced fetal growth. 166 term births had complete blood counts (CBC) and differentials performed on cord blood. Of these, 139 had placental examination blinded to hematologic data. Total nRBC count was calculated from nRBC count/100 white blood cells (WBC) and corrected WBC count. Nonparametric (Spearman's) correlations assessed associations with hematocrit and total RBC, reticulocyte, and neutrophil counts and with placental parameters. Logtransformed nRBC counts served in multivariate regression. Our results were that nRBCs and reticulocytes were correlated (p = 0.03, r = 0.21). nRBC and reticulocytes did not correlate with hematocrit. After adjustment for reticulocyte count, nRBCs were correlated with band neutrophils (p = 0.02, r = 0.30). Reticulocytes correlated with neither myeloid count. nRBC count was related to birthweight (r = 0.21) and placental weight (r = 0.20), but not to other placental measures. Reticulocyte count was related to placental volume (r = 0.20, p = 0.02) and fetal/placental weight ratio (r = −0.31, p = 0.007). No placental pathology was related to fetal hematology. A predictive equation including birthweight and placental weight showed p = 0.05, although each individual p was >0.4. In conclusion, our data suggest that elevated nRBC and reticulocyte counts identify clinically well term fetuses with compensatory responses to altered [O₂]. A portion of nRBC variance is independent of reticulocyte count, and attributable to change in band neutrophil count. In well term newborns, nRBC count is related to birthweight and placental weight, but this relationship is likely complex and non-linear.