After ovulation, when the sperm and egg meet to form a fertilized egg, the fertilized egg gradually moves from the fallopian tube to the uterine cavity. At this time, when the fertilized egg moves to the uterine cavity, the endometrium is fully prepared and the estrogen and progesterone levels are correspondingly high. The fertilized egg will implant in the endometrium. When a blood HCG test is performed 1-2 days after the implantation of the fertilized egg into the endometrium, it is possible to detect a significantly high level of HCG, which is different from the non-pregnant state, so a pregnancy detected at this time proves that the embryo has been implanted.