OBJECTIVE To determine the distribution of fetal erythroblasts in the maternal circulation at different gestations. METHODS Maternal blood was obtained from 152 normal singleton pregnancies at 11-40 weeks of gestation. Fetal erythroblasts were isolated using triple density gradient separation and anti-CD71 magnetic cell sorting techniques. The enriched erythroblasts were stained with Kleihauer-Giemsa and with fluorescent antibodies for the zeta (zeta), epsilon (epsilon) and gamma (gamma) globin chains. The percentage of fetal cells positive for each stain was calculated. Fluorescence in situ hybridization for X and Y chromosomes was also performed. Comparison was made in the proportion of positive fetal erythroblasts among the different gestational ages. RESULTS The proportion of erythroblasts stained positive with gamma-globin chain and Kleihauer-Giemsa decreased with gestation from a median of 2% at 11 weeks to 0.5% at 40 weeks. Similarly, there was a decrease in the percentage of Y-signal-positive cells from 1% at 11 weeks to 0.3% at 40 weeks. The proportion of enriched fetal erythroblasts stained positive with zeta- and epsilon-globin chains decreased exponentially from respective medians of 0.6% and 1.5% at 11 weeks to zero after 19 weeks and 24 weeks. CONCLUSIONS In normal singleton pregnancy the percentage of fetal erythroblasts enriched from maternal blood decreases with gestation.