Failure to double HCG in the early stages could be a poorly developed embryo or ectopic pregnancy. HCG, also known as human chorionic gonadotropin, is secreted by the trophoblast cells of the embryo. In early pregnancy, this hormone should double every 48 hours, and if this is not achieved, it means that the embryo is secreting less of the hormone, and this condition may be ectopic pregnancy. Ectopic pregnancy refers to the fertilized egg in the fallopian tube, the fallopian tube is relatively small, so the concentration of HCG secreted by the embryo is relatively low, and can not reach the doubling conditions. If the embryo itself is not well developed, the hormone secreted is relatively low, and the conditions for doubling are also not met.