The main cause of a high risk of trisomy 21 is basically genetic. trisomy 21 is a type of Down’s syndrome screening, and sometimes a high-risk fetus may not necessarily be born with a congenital disease, because this screening has a large margin of error. it is a population-based screening and is determined by the level of risk, not an absolute value. The presence of a high risk of trisomy 21 can also be caused by a virus, such as the rubella virus, during pregnancy, or by exposure to outside chemicals, or by a genetic predisposition from both spouses. Therefore, when this condition is detected, an amniocentesis is required to further diagnose the condition.