The third day of pregnancy is a process in which the sperm and the egg are combined and the fertilized egg has not yet attached to the bed, so it cannot be detected. The earliest way to detect pregnancy is to test the HCG value in the blood in the vein ten days after intercourse, if there is a significant increase in the HCG value, it can be diagnosed as pregnancy. The use of pregnancy test paper and ultrasound to detect pregnancy should wait until after the menstrual period, especially one week after the normal menstrual period, the test results are more accurate, and ultrasound to detect pregnancy can also rule out the possibility of ectopic pregnancy. It is not recommended to test three days after sexual intercourse during the preparation period. It is recommended to keep a good mindset, pay attention to the intake of nutrients, pay attention to rest, and avoid exertion, so as to create conditions for successful conception.