A single red line on the twelfth day after intercourse does not mean that you are not pregnant. It is possible that the time period is relatively short and the concentration of HCG in the urine is low, so you can wait for 1 week before taking another test. Generally speaking, you can use pregnancy test paper to test for pregnancy about 14 days after intercourse during ovulation, as the HCG in the urine may start to increase at this time. So day 12 is still relatively early, and it’s possible that you won’t be able to test. However, pregnancy test paper is not completely accurate, if you want to be as soon as possible or more accurate, it is recommended that you can go to the hospital about 10 days after intercourse to draw blood to check HCG to determine whether you are pregnant. If there are other abnormalities, it is recommended to seek timely medical attention