Prenatal diagnosis on fetal cells from maternal blood: practical comparative evaluation of the first and second trimesters.

Objectives- Several attempts have been made to determine the gestational period in which the maximum number of fetal cells can be found in maternal blood and consequently which is the best week in which to perform a reliable non-invasive prenatal diagnosis. Most studies conclude that the number of nucleated red blood cells (NRBC) increases in line with gestation, but the number of cells that are fetal in origin (FNRBC) decreases in the third trimester. The aim of the present study was to make a practical comparative evaluation of the first and second trimesters to ascertain the period in which a greater number of FNRBC can be found of the total number of NRBC identified. Methods- Double density gradient and a posterior positive selection (CD71) by magnetic activated cell sorting (MACS) were employed. In the final fraction, erythroblasts were identified using Kleihauer staining and were studied using the fluorescence in situ hybridization (FISH) interphasic technique. Results- There was a significant difference (p<0.05) between the mean number of FNRBC found in the first and second trimesters. Conclusions- The number of FNRBC increases from the first to the second trimester. It appears that the optimum week in which to perform a reliable non-invasive prenatal diagnosis is around the 15th week.