1. Less weeks of pregnancy

　　The time of the last period may have been misestimated, resulting in a discrepancy between the actual gestational age and the estimated gestational age.

　　2. Non-single fetus

　　It could be twins or multiple births, as the number of multiple pregnancies is higher than that of single pregnancies.

　　3. Mole

　　It may be an abnormal pregnancy - hydatidiform mole, in which blood HCG values are usually higher than normal pregnancy values for the corresponding gestational week. If hydatidiform mole is confirmed, immediate hospitalization is required.

　　4. Injected WITH HCG to protect the fetus

　　The median multiple reference range of -HCG on the down's screening list is 0-2.5. If it is higher than that, it will have an adverse impact on the safety of mother and child during pregnancy, such as the risk of pregnancy-induced hypertension. Some pregnant women have been injected with HCG in the early stage of pregnancy to avoid pregnancy, so there is a possibility that the HCG value may be higher when tang sieving.