Home pregnancy test kits are designed for a single use and cannot be reused. The home pregnancy test has a special platform that is treated with an antibody that will bind with the hormone HCG in a pregnant woman's urine; if the hormone is present, a change will occur in the testing window. This is usually a color change created by a pigment molecule that is only triggered by HCG to form a line, according to About.com.
The pregnancy hormone is more concentrated in early morning urine, which is why most tests come with the caveat that the most accurate results are achieved if the test is taken with morning urine. Most women produce enough HCG on the sixth day after conception to trigger a positive test result, although this is not always the case.