how does a pregnancy test work?

Chandy said:

i know it tests for b-hCG in the urine but HOw does it detect it and show a positive result?

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Found this by googling "how work pregnancy test" - from about.com:
The tests work by binding the hCG hormone, from either blood or urine, to an antibody and an indicator. The antibody will only bind to hCG; other hormones will not give a positive test result. The usual indicator is a pigment molecule, present in a line across a home pregnancy urine test. Highly sensitive tests could use a fluorescent or radioactive molecule attached to the antibody, but these methods are unnecessary for an over-the-counter diagnostic test. The tests available over-the-counter versus obtained those at the doctor's office are the same. The primary difference is the decreased chance of user error by a trained technician. Blood tests are pretty much equally sensitive at any time. Urine tests tend to be most sensitive using urine from early morning, which tends to be more concentrated (would have the highest levels of hCG).