Why Tang Screen is a critical risk?

When I went to the hospital for Down's screening, the doctor told me that the result was a borderline risk. I wonder why Tang's screening is a borderline risk?

Tang screening is a critical risk, indicating that the probability of the fetus having Down syndrome increases, but it cannot be clearly determined that the fetus has chromosomal abnormalities, so pregnant women should go to the hospital for Tang screening when there is a critical risk Further examination, such as non-invasive DNA examination, amniocentesis, etc., can clearly understand whether the fetus has 21-trisomy syndrome, 18-trisomy syndrome and other diseases. If the test results indicate that the probability of a Down syndrome child is very high, the pregnant woman can terminate the pregnancy in time under the doctor's advice to avoid the birth of a Down syndrome child. Pay attention to go to the hospital for regular check-ups during pregnancy to understand the baby's development.