English: There are three different sites on a typical urine test for pregnancy: the Reaction Site, Test Site, and Control Site. At the first site, monoclonal antibodies are used to trap specific hormones that indicate pregnancy, while fixed antibodies are used at the last two sites. At the Reaction Site, the hormone hCG is trapped by the monoclonal antibodies. The hormone hCG is produced at high levels by a pregnant woman's placenta, which is why it is used to depict positive results for these kinds of tests. At the Test Site, the trapped hCG is sandwiched in between a fixed antibody with a dye substrate, resulting in the first line of the pregnancy test appearing. This line will only appear if the result of the test is positive. At the Control Site, a free antibody in the bloodstream of the urine is trapped by the fixed antibody with the dye substrate, resulting in the second line on the pregnancy test appearing. The purpose of this line is to indicate that the pregnancy test is valid and accurate. If this second line is the only line to appear, then the test result is negative. However, if both lines (the first and second) were to appear, then the result of the pregnancy test is positive.