Menstrual period is menstruation. To determine whether the bleeding on the 12th day of ovulation is due to implantation or menstruation, you can differentiate from the blood HCG level, the amount of bleeding and the duration of bleeding. 1. Blood HCG level: If the bleeding on the 12th day of ovulation is due to implantation, the blood HCG test will usually show a positive result, while if it is due to menstruation, it will usually show a negative result. 2. Bleeding volume: Bleeding at the time of implantation is caused by implantation of the fertilized egg into the uterine lining, and the bleeding volume is small; while bleeding during menstruation is caused by shedding of the uterine lining, and the bleeding volume is relatively large. 3. Bleeding duration: if the bleeding on the 12th day of ovulation is due to implantation, the bleeding usually lasts only 1-2 days, while menstrual bleeding is longer and usually lasts 3-7 days. If a woman has had intercourse before or after ovulation and has not used contraception, then the possibility of implantation is relatively high, and it is recommended to go to the hospital as soon as possible for blood HCG measurement.