Association of ABO incompatibility with elevation of nucleated red blood cell counts in term neonates

OBJECTIVE: Nucleated red blood cells in the circulation in term neonates have been associated with a wide range of pathologic conditions. We sought to examine the relationship between nucleated red blood cells in the circulation of term neonates and maternal-neonatal blood type compatibility. STUDY DESIGN: We prospectively collected umbilical blood from all live-born neonates delivered at our institution. Venous blood was analyzed for nucleated red blood cells and is reported as the number of nucleated red blood cells per 100 white blood cells. We reviewed maternal and neonatal records for neonates born at greater than or equal to 37 weeks' gestation for correlative clinical data. Statistical analysis was performed with the SAS statistical software package (version 6.12; SAS Institute, Inc, Cary, NC). Kruskal-Wallis analysis was used as a nonparametric test. RESULTS: We evaluated 1661 neonates delivered during the study period and found a mean (+/- SD) of 9.29 +/- 18.56 nucleated red blood cells per 100 white blood cells (range, 0-327 nucleated red blood cells per 100 white-blood cells). Nucleated red blood cell counts were lower in ABO-compatible maternal-fetal dyads (mean:+/- SD, 8.29 +/- 12.84 nucleated red blood cells per 100 white blood cells; range, 0-216 nucleated red blood: cells per 100 white blood cells) than in ABO-incompatible dyads (mean +/- SD, 13.16 +/- 13.16 nucleated red blood cells per 100 white blood cells; range, 0-327 nucleated red blood cells/100 white blood cells; P=.006). Neonates of mothers with blood groups A and B had significantly lower nucleated red blood cell counts (P<.05). Dyads with maternal type O and neonate type B had significantly higher nucleated red blood cell counts (P <.002). Nonparametric testing determined that type O mother and type B neonate combinations had significantly higher umbilical cord nucleated red blood cell counts (P<.001). CONCLUSION: Maternal-fetal ABO incompatibility is associated with elevation of nucleated red blood cell count in term neonates. Nucleated red blood cell elevation does not always connote a serious pathologic process, however, because ABO incompatibility usually does not adversely affect neonatal outcome. The clinical significance of an elevated nucleated red blood cell count thus is limited.