Down screening is a method to initially assess the risk factor of Down syndrome in fetuses during pregnancy, and some pregnant women do not easily pass it during pregnancy, commonly in the following cases: 1. older age: if the age of pregnancy is more than 35 weeks old or the age of due date is more than 35 years old, the Down screening test is prone to abnormal results, because with increasing age, ovarian function declines, egg quality is poor, and chromosomal variants are likely to occur, leading to Down’s syndrome may occur; 2. Heavy weight: Generally, a body mass index of more than 25 is prone to abnormal results of the Down’s screening test; 3. Inconsistent gestational weeks: Some women have irregular menstruation, and the gestational weeks and the time of menopause do not match, which may cause errors in the calculation results and abnormal results. In this case, it is usually recommended to bring the ultrasound when the intrauterine pregnancy is just confirmed, and ask the doctor to verify the gestational week with the size of the gestational sac or the size of the fetal bud; 4, heavy environmental pollution: if the pregnant woman is often exposed to radiation or chemicals before pregnancy, and lives in a heavily polluted area, it is easy to have embryonic chromosomal variation, and the Down screening result is not easy to pass; 5, bad maternal history: if there are many previous embryonic abortions or births If the fetus has had chromosomal abnormalities before, it is easy for the fetus to have chromosomal abnormalities again; 6. Pregnant women have chromosomal abnormalities: if the pregnant woman has chromosomal abnormalities herself, she may inherit such abnormalities to the fetus, which may also lead to abnormal results of the Down screening test.