Timing of fetal nucleated red blood cell count elevation in response to acute hypoxia

We determined the effect of an acute episode of severe hypoxia on peripheral nucleated red blood cell (RBC) counts in the fetal rat. Timed pregnant rats were randomized to a 2-hour exposure to hypoxia (placement in a chamber containing a gas mixture with 9% O₂ +3% CO₂ + balanced N₂) or to a 2-hour exposure to a sham chamber containing room air. Two maternal animals per group underwent cesarean section immediately after the 2-hour period and then 4, 12, 24, 36, 48, and 60 h after exposure. Fetal nucleated RBC counts were compared between groups at each time interval. The nucleated RBC counts were not significantly different in the hypoxia group until 12 h (mean ± SEM 158.0 ± 22.4 RBC/10 high-power fields vs. 90.6 ± 11.0; p = 0.03) and 24 h (133.2 ± 16.0 vs. 84.1 ± 9.0; p = 0.04) after exposure. There were no differences between groups 36, 48, and 60 h after exposure. In the near-term rat fetus, acute hypoxia was associated with a delayed but transient increase in peripheral nucleated RBC counts.