Chromosomal aberration is a phenomenon occurring relatively commonly in the development process. Chromosomal aberration is known to have various causes, and its frequency has been reported to vary particularly according to maternal age. Though different among reports, the frequency was around 0.6-0.8% in analysis with all childbirths1 and around 2-3% in case childbirths were reclassified based on maternal age of 35.2 In addition, the frequency increased when the maternal age was relatively young, artificial insemination was used, etc.3 Spontaneous abortion related to such chromosomal aberrations occurs usually in the first trimester, and consequently, the prevalence of trisomy 21, 18 and 13 decreases with the advance of gestational age.4 Chromosomal aberrations that can be found in prenatal diagnosis are largely divided into aneuploidy, structural aberration, mosaicism, uniparental disomy, small defect, etc., and additionally, there can be marker chromosomes. Marker chromosomes are observed in around one out of 1000 cases of chromosomal analysis, and are known to be related to old maternal age.5 It is reported that around 80% of marker chromosomes detected are new ones, and 13% of marker chromosomes are associated with physical abnormalities or cognitive disorders.6 The frequency and type of chromosomal aberration are known to be different depending on the time of evaluation, population group under the analysis of structural chromosomal aberration, and the banding level of the laboratory performing cytogenetic evaluation.7 Thus, it is increasingly important to conduct adequate prenatal genetic screening and analyze its results properly. Among chromosomal aberrations detected in pregnant women who received amniocentesis because of their age, 64% were trisomy (21% trisomy 21), 11% translocation, 17% sex chromosomal aberration, and 8% other types of chromosomal aberration.2 Thus, considering the increasing number of old age pregnancies in Korea, careful attention should to paid in analyzing the results of prenatal genetic tests. Thus, this study purposed to discuss how to analyze the results of maternal serum marker tests and sonography used in prenatal genetic evaluation.