If it is a normal period, pregnancy is not detected on the day of the period. If there is vaginal bleeding when you are about to have your period and then the HCG test is positive either in urine or blood, this is not a normal period, it is a pre-eclampsia or spontaneous miscarriage, and there is also a condition called biochemical pregnancy, which is an unsuccessful pregnancy. In this case, you can come back in a few days to see if the HCG level is rising or falling, and to observe if the vaginal bleeding is increasing or decreasing. If the HCG drops, it means that the pregnancy has been aborted and you can treat this bleeding as a normal period. The next month, if you are not pregnant, you will also have your period again in about a month. If the HCG is rising, there is no miscarriage. You should also consider the possibility of ectopic pregnancy, and you need to review the ultrasound to see if there are similar gestational sacs inside and outside the uterus, or if there is no echo in the uterus. In short, if the period is normal, pregnancy cannot be detected. If pregnancy is detected, it is not normal menstruation.