The ability to detect pregnancy after two weeks is due to the fact that when the sperm and egg unite to form a fertilized egg, it runs in the fallopian tube. After a week it reaches the uterine cavity, where it lays and grows. The trophoblast cells of the embryo secrete human chorionic gonadotropin, which can be detected in the blood of the pregnant woman as early as 7 days after fertilization and usually in the urine in about half a month. The blood concentration usually reaches its peak at 8-10 weeks of pregnancy and decreases rapidly after 10 days. So under normal circumstances, if you have sex for two weeks can be detected, want to be accurate it is best to go to the blood test, not only to detect whether it is negative or positive, but also to see the specific number.