All tests for self-determination of pregnancy work on the same principle. They determine the level of human chorionic gonadotropin (hCG), a pregnancy hormone, in the urine.
In the first days after fertilization (usually within 6–7 days), hCG levels rise rapidly, doubling every two days. The purpose of the test is to establish the level of hCG. This is where the hormone sensing reagent is colored. The same second line appears on the test, which indicates fertilization.