Pseudomosaicism has been observed more often in amniotic fluid than in any other tissue. It was reported that the cases with Pseudomosaicism were occupied with almost 7%. Therefore, In situ method was proposed for distinguishing to truemosaicism from pseudomosaicism in cytogenetic analysis of amniotic fluid. Two more cells that the same structural abnormality in chromosome are defined as clone. In situ method is more advantageous to detect pseudomosaicism precisely because the colony is analyzed in the method. According to the statistics of cytogenetic analysis performed at department of clinical pathology, Sam sung Medical Center for 1998, the cases with pseudomosaicism were occupied with 3%. However karyotype reanalyzed through resampling amniotic fluid of them was all normal. In conclusion, In situ method is thought to be useful in detection of pseudomosaicism.