If the test is negative on the 13th day of intercourse, it does not mean that you are not pregnant. If the sperm and egg united on the same day of intercourse, it takes 15 days at the latest for the fertilized egg to enter the uterine cavity and take hold, so it is possible that the fertilized egg has not yet taken hold at the 13th day. If there is still no menstruation after 5 days, you can check whether the blood HCG is increased by means of early pregnancy test paper or preferably by taking a blood test, if the blood HCG value is >10IU/mL, it indicates pregnancy. If the blood test is still negative after the 15th day of intercourse, it is clinically unlikely to be pregnant. Therefore, if pregnancy is not detected on the 13th day of intercourse, it does not necessarily mean that you are not pregnant, and you should be retested after some time.