There are three possible scenarios for high HCG levels: pregnancy, gestational trophoblastic disease, and tumors. 1. Pregnancy: trophoblast cells begin to produce HCG after the fertilized egg is deposited, and it can be detected after 1 week of pregnancy, and reaches the highest value (50,000-100,000 IU/L) in the 8th to 10th week of pregnancy, and then decreases rapidly after 1 to 2 weeks, and then gradually decreases until it disappears after delivery. 2. Gestational trophoblastic diseases: common ones are gravidarium, erosive gravidarium and choriocarcinoma, in which trophoblasts are pathologically highly proliferated and produce a large amount of HCG, which can be more than 100,000IU/L. 3. Tumor: As some tumor cells can also secrete HCG, common ones are lung cancer, gastric cancer, liver cancer, ovarian cancer, cervical cancer, etc. The larger the size of the tumor is, the higher the HCG level will be. If HCG is elevated in clinic, one should go to the hospital for complete examination to clarify the cause of the disease and timely treatment.