Objectives: Incorrect segregation of chromosomes 13 and 21 occurs frequently in human preembryos. Detection of these aneuploidies before preembryos are transferred to women of advanced maternal age may have great significance in human assisted reproductive medicine.Design: To compare the results of aneuploidy for detection of chromosome 13 and 21 by different probes and procedures.Materials and Methods: A total of 334 blastomeres from arrested preembryos (n = 92, averaging 3.6 cells/preembryo) were obtained from consenting IVF patients. In the first FISH procedure, numerical analysis of chromosomes 13 and 21 was performed in a 4-color FISH (no. of blastomeres = 172) with a combined 1321 FISH probe (alpha satellite DNA probe, Oncor), together with 3 other specific probes (X, Y, and 18, Vysis). In the second protocol, chromosomes 13 and 21 were determined separately (no. of blastomeres = 189) by 13 and 21 specific probes (locus specific probes of 13 and 21 mixture, Vysis).Results: With the first protocol, FISH signals were obtained in 124 of 172 cells (72.1%), in which 47.1% (81/172) of the blastomeres were diagnosed as normal (all 4 signals). The distribution of abnormal 1321 chromosomal signals was 5.2% without signal (n = 9), 3.5% one signal only (n = 6), 8,1% two signals only (n = 14), 4.1% three signals only (n = 7) and 4.1% more than four signals (n = 7). In the second procedure, 127 of 162 cells (8.4%) gave rise to FISH signals, of which 50.6% (82162) of the cells had a correct number of signals (two signals each for chromosomes 13 and 21). In addition, 4.9% (n = 8) showed monosomy 13 and trisomy 21, and 1.2% (n = 2) were trisomic for 13 and monosomic for 21. These monosomic and trisomic cells would have been diagnosed as normal ones if the first protocol had been used. The remaining 37 abnormal nuclei (22.8%) were diagnosed as follows: 1 13 & 0 21 (n = 1); 1 13 & 1 21 (n = 3); 1 13 and 2 21 (n = 2); 2 13 and 0 21 (n = 3); 2 13 & 1 21 (n = 9); 2 13 & 3 21 (n = 4); 3 13 & 2 21 (n = 2); 3 13 & 3 21 (n = 2); 4 13 & 2 21 (n = 3); 4 13 & 3 21 (n = 2); and 4 13 & 4 21 (n = 6).Conclusions: Our results indicate that using a dual-color FISH to estimate individual chromosome composition of 13 and 21 has a great advantage over a single colored 1321 alpha satellite probe, in that not only a more precise diagnosis was made in terms of each chromosome, but also fewer missing signal were observed, thus increasing the sensitivity and accuracy of aneuploidy detection in preimplantation preembryos.