Abstract: With the advent of cell-molecular biology, prenatal screening and diagnostic methods have also changed, and more data need to be interpreted correctly，which is a challenge for prenatal diagnosis doctors. Among them, chromosome mosaicism is one of the most important factors that affect the accuracy of prenatal screening and diagnosis. Chromosome mosaicism is prevalence in human and affected by many factors, such as mother, father, placental and other factors. The main reason for the formation of mosaicism is  chromosomal error separation in mitotic. The clinical manifestations of mosaicism are affected by a variety of factors, and their clinical outcomes are unique to each individual. The effect of mosaicism on prenatal screening and diagnosis are mainly in two aspects: ①Screening of cell-free fetal DNA (cfDNA) in maternal blood. Although a large number of related literature reported its higher detection rate for 21,18,13 trisomy, while cfDNA from the placenta, and confined placental mosaicism is an important factor affecting the accuracy of the results. ②If the chronic karatype is mosaicism，amniocentesis is required for futher confirmation. In conclusion, understanding the origin, mechanism and clinical outcome of chromosome mosaicism correctly, can provide patients with more valuable genetic information, and it′s importance for providing genetic counseling in prenatal screening and diagnosis.

Key words: Prenatal diagnosis, Chromosomes, Chimera, Amniocentesis, Prenatal screening