Trisomy 20 is the most common autosome mosaicim diagnosed from genetic amniotic fluid and placental tissue(or CVS). Only rarely 100% trisomy 20 was identified of cells culture from the amnion and placental tissue. But not in fetal tissue and blood cell culture. Complete trisomy 20 is not viable. Outcome of prenatally detected trisomy 20 mosaicism is normal phenotype in 90-95 % of cases. Abnormal phenotype has been in 5-10% of the reported cases of trisomy 20 detected on amniocentesis. Abnormal outcomes found include unexplained fetal demise, intrauterine growth restriction(IUGR) and multiple congenital anomalies. It remains a dilemma in diagnosis and counselling. There are many case reports from abroad on trisomy 20, some of which involve long-term follow-up and outcome, but it IS hard to find out such reports in Korea. We report on a domestic case of trisomy 20 which we observed in amniotic fluid and placental tissue cultures. but trisomy 20 has never been detected in cord blood, fetal blood and fetal tissue cultures.