Bleeding 12 days after ovulation may be implantation bleeding or menstrual flow. 1. Bleeding on implantation: If you have had unprotected sex before or after ovulation, it may be bleeding on implantation. Bleeding on implantation usually occurs about one week after the formation of the fertilized egg, at which time the fertilized egg implanted in the uterus and stimulated the uterine lining, resulting in part of the uterine lining peeling off, which causes bleeding. 2. The onset of menstruation: generally about 14 days after ovulation is the menstrual period, which may be advanced or delayed for about 3 to 5 days, so bleeding 12 days after ovulation may also be the onset of menstruation, that is, not pregnant, but menstruating. If bleeding 12 days after ovulation is suspicious of pregnancy, you can seek medical attention for tests related to early pregnancy.