A blood test for blood HCG on the twelfth day of ovulation can detect if you are pregnant. If you have intercourse on the day of ovulation, the sperm and egg will unite to form a fertilized egg, which will enter the uterine cavity in as early as nine days and then free for two to three days before it will be laid. Therefore, on the twelfth day of ovulation, the ovum may have already been fertilized and a blood test for HCG will show a positive result, although it is better to test on the fifteenth day after ovulation for a higher positive rate. If the test is done by urine, it is recommended to test on the twentieth day after ovulation. It is also recommended to use the morning urine test for a higher positive rate, and if the first test has a negative result, it is recommended to do another test every three days.