There are cases when a test paper cannot detect pregnancy, if the time after intercourse is too short to detect it with a test paper. This is due to the fact that when the sperm and egg unite to form a fertilized egg, it takes about a week for them to reach the uterine cavity, and they can be detected in the woman’s blood only on the second day after they have been laid. If you use test strips, you are testing the urine, which has a low level of hormones, and under normal circumstances, it will not be detected until half a month after intercourse. If the test is done around 10 days after intercourse, it may not be detected, in which case it may appear as a false negative. In order to test the accuracy of the test paper, it is best to test half a month after intercourse, preferably with the first urine in the morning.