If the menstrual cycle is less than 25 days, the blood HCG is about 500mIU/mL when the pregnancy is 28 days old. If the menstrual cycle is 30 days old, the blood HCG is often not able to detect positive results when the pregnancy is 28 days old, and it is recommended that the test be done after 3 days, so that the positive rate will be increased. After all, only when the fertilized egg enters the uterine cavity and settles in the uterine cavity can it lead to an increase in blood HCG, so the fertilized egg may not have settled in the uterine cavity on the 28th day of pregnancy, and if you do the relevant tests a few days later, the positive rate will be higher. Because it takes about 0.5 months from the day of intercourse for the fertilized egg to enter the uterine cavity, it is best to take a blood test for HCG 31-32 days after menopause.