Final answer:
The substance causing a positive home pregnancy test is the hormone human chorionic gonadotropin (hCG), which is produced following the implantation of a fertilized egg in the uterus and is detected by the test line on a pregnancy test.
Step-by-step explanation:
The substance that causes a positive home pregnancy test is called human chorionic gonadotropin (hCG), which is a hormone produced after a fertilized egg attaches to the wall of the uterus. This hormone can be detected in urine and blood, and it is the specific marker that over-the-counter pregnancy tests are designed to identify. When a woman takes a home pregnancy test, the ability of the test to detect hCG in her urine will determine if the result is positive, indicated often by two colored lines on the test device.
These tests usually include a control line to validate that the test is working correctly, and a test line that only appears if hCG is present in the urine, signaling a positive result. The test line appears due to a reaction between the hCG hormone and an antibody specific to this hormone embedded in the test strip. When hCG is present, it binds to the antibody, a secondary antibody that carries a colored particle also binds to the complex, and this sandwich causes the line to appear.