Fetal nasal bone is not shown at 18 weeks of pregnancy, non-invasive DNA test is normal, in this case, you can not do amniocentesis first, and then do ultrasound to review to see if it is necessary to do amniocentesis. At 18 weeks of pregnancy, ultrasound examination found that the fetal nasal bone does not show, this does not necessarily mean that the fetus has an abnormality, because this examination result may have something to do with the fetal position. If the non-invasive DNA test does not reveal any abnormality, it is relatively unlikely that the fetus has a chromosomal abnormality. This is because the accuracy of non-invasive DNA testing is relatively high, with detection rates of 99%, 97% and 91% for trisomy 21, trisomy 18 and trisomy 13 screening respectively. Now that the fetal nasal bone is not visible on ultrasound and the non-invasive DNA test is normal, you can have another ultrasound in a while to recheck it. If the nasal bone is still not visible in repeated ultrasounds, then the possibility of chromosomal abnormalities in the fetus should be considered and amniocentesis is recommended.