Pregnancy tests work by measuring the body’s level of human chorionic gonadotropin (hCG) in either urine (the most common method) or blood. A woman’s body begins producing the hCG hormone after a fertilized egg implants in the lining of the uterus.
The concentration increases rapidly during early pregnancy, doubling every two to three days, so the timing of testing is key. If you test too early, there might not be enough hCG for detection. If you test after your first missed period, the result will be more accurate.