How can urine be used to detect pregnancy?

The hormone Human Chorionic Gonadotrophin (HCG) starts to be secreted six days after conception from a developing embryo. A pregnancy test stick has three bands. In the first are monoclonal antibodies which are specific to HCG and are associated with a pigment activating- enzyme. When placed in urine, the fluid moves up the test strip passing first band, allowing any HCG present to bind to these antibody-enzyme complexes. As the complexes (also bound to HCG) move up to the second band (the test zone), another enzyme will bind to each complex on one of HCG’s 5 binding sites forming a sandwich assay. The assay sticks to the test band allowing the enzyme molecule to activate the dye and a blue line is produced. If HGC isn’t present, an assay wont form and no dye will be released. Any unbound enzyme-complexes (bound to HCG) from the first strip will end up in the last band, the control, and should activate more dye. if not, the test was faulty. A positive test therefore should give two blue lines and a negative, none.