The normal value of trisomy 18 is 1:350, as long as the test result is less than the risk cut-off value, it is within the normal range, if it is higher than 1:350, it means the fetus has the possibility of malformation, and further examination by amniocentesis is needed to confirm the diagnosis. Trisomy 18 is the second most common chromosome after congenital stupidity and is recommended to be done between 16 and 21 weeks. It is a very important test during pregnancy, so pregnant mothers should not miss the test.