Failure of non-invasive DNA testing is mainly related to abnormalities in blood samples, abnormally high maternal DNA content, and low fetal DNA content. 1. Abnormal blood samples: Blood specimens need to be collected in accordance with standard procedures and processed and stored in a timely manner within 8 hours. Free DNA is present in maternal blood during pregnancy, with about 5% to 20% coming from the fetus and most of the rest originating from the mother. If the blood specimen is left for too long before separation, or if it is improperly stored with violent shocks, rupture of maternal blood cells occurs and maternal DNA is released into the blood specimen, and the percentage of DNA from the fetus decreases. 2. Abnormally high maternal DNA content: when the mother has health abnormalities, such as gestational hypertension and other gestational diseases, the DNA from the mother will increase a lot, resulting in a decrease in the proportion of DNA from the fetus. 3. Low level of fetal DNA: Low level of free fetal DNA in maternal blood may occur when the gestational week is less than 12 gestational weeks, or even if it is more than 12 weeks, but there is IVF, twin births, and maternal over-obesity. In addition, non-invasive DNA testing should be performed in a regular medical unit and the timing of the test should be chosen appropriately under the guidance of a doctor.