Home pregnancy tests are kind of magical—they’re like those litmus test things from junior high science, except they can tell you whether you’ve got a baby in there. These tests work by detecting trace levels of the pregnancy hormone human chorionic gonadotropin (hCG) in the urine; hCG is present after egg implantation, which occurs six to 12 days after fertilization, and is secreted by the cells that are beginning to form the placenta.

Source: 9 Historical Methods of Detecting Pregnancy | Mental Floss UK