Down screening for trisomy 21 is a critical risk that can be caused by fetal chromosomal abnormalities. Down screening is short for prenatal screening test for Down syndrome. Down screening 21 trisomy critical risk, which means the fetus has the possibility of Down’s syndrome, need to do further amniocentesis or non-invasive DNA test to confirm the diagnosis. The main cause of Down’s syndrome 21 critical risk is a possible chromosomal abnormality in the fetus, which is a congenital developmental abnormality. Down’s syndrome is not a diagnosis and is a critical risk, not a high risk, so don’t worry too much. Keep a good attitude, further examination to confirm the diagnosis, there is no problem with the fetus can continue the pregnancy.