Pregnancy cannot be detected 4-5 days after intercourse because the egg has just been fertilized and has not yet reached the uterine cavity and is still wandering inside the fallopian tube, so it has not yet been deposited, and the trophoblast layer of the embryo has not yet produced chorionic gonadotropin, which cannot be detected in this case. It is only when the embryo reaches the uterine cavity about 6 days after the intercourse, and it is implanted in the uterine cavity, that the trophoblast cells of the embryo will secrete chorionic gonadotropin on the second day of implantation. Chorionic gonadotropin will be detected in the woman’s blood, so the earliest time it can be detected in the woman’s blood is 1 week after intercourse, and it cannot be detected too early.