Threatened miscarriage refers to the signs of a potential miscarriage, but this does not necessarily mean that a miscarriage will definitely occur or is inevitable. Whether HCG levels decline during a threatened miscarriage depends primarily on the cause of the threatened miscarriage and the ultimate outcome. First, if the gestational sac is developing well and it is only a mild case of threatened miscarriage, the HCG levels will not decrease and will continue to rise. Second, if the threatened miscarriage is caused by poor embryo development or embryonic arrest, the HCG levels will naturally decline. Third, if the condition of the threatened miscarriage is severe and eventually leads to a miscarriage or inevitable miscarriage, the HCG levels will also decline. Therefore, whether HCG levels decline during a threatened miscarriage largely depends on the cause and the severity of the condition.