Non-invasive DNA is an examination item to screen the incidence of fetal chromosomal diseases during pregnancy 16~20 weeks. This method takes maternal blood, extracts fetal free chromosomes, cultivates them for 7~ 10 days, and then analyzes the incidence of fetal 2 1 trisomy, 18 trisomy and 13 trisomy, with an accuracy rate of over 99%. If there are high-risk factors, further amniocentesis is needed to confirm the diagnosis.