If the HCG doubles quickly but the gestational sac is not visible in the uterine cavity, it is possible that the fertilized egg has just settled in the uterine cavity and the gestational sac is not visible on ultrasound. If the blood HCG is doubled and the gestational sac is not visible, the clinical observation should be dynamic, mainly through ultrasound examination to understand the location of the gestational sac, and it is also recommended to continue to monitor the blood HCG and combine the blood HCG and ultrasound results to give a comprehensive analysis.