Prenatal diagnosis of hereditary diseases is usually performed by collecting fetal samples using amniocentesis, chorionic villus sampling (CVS), or cordocentesis. However, these procedures are associated with some degree of risk. For example, abortion owing to hemorrhage or infection occurs in 0.2-0.4% of patients undergoing amniocentesis. Furthermore, CVS reportedly presents the potential risk of fetal limb malformation in 0.01-0.03% of cases (1). Each method has the risk of misdiagnosis because of contamination by maternal cells. Thus, the development of a suitable noninvasive method is important (Table 1). Table 1 Summary of Prenatal Diagnostic Methods Diagnostic methods Weeks of gestation Benefits Defects Fetal loss rate,% Amniocentesis 15-18 Relatively simple Necessary to culture 0 2-0.4 Contamination by maternal cells Chorionic 9-12 Early and direct Limb malformations 0 5-10 villus sampling diagnosis possible (0 01-0.03%) Contamination of maternal cells Placental mosaicism Cordocentesis 18- Early and direct Bleeding 0.5-2.0 diagnosis possible Fetal bradycardia Contamination of maternal cells Maternal 6- Noninvasive - 0 peripheral blood.