How does a pregnancy test work?

When pregnant, placenta of a woman secrets a specific hormone, called hCG (human chorionic gonadotrophin). hCG maintains progesterone production that is critical for a pregnancy in humans.

Through circulation, hCG accumulates in urine. Once the level of hCG reaches a certain level, it will be detectable by the test strip which will show a positive result to announce your pregnancy.

A test strip is equipped with a type of special reagent which reacts and binds to hCG. While urine and dye touches the reagent area, hCG will bind with the reagent and dye, and thus shows a positive result of pregnancy. Very few factors have impact on the result, but correct operation is very important to avoid erroneous result.

Note, urine test is easy to do anywhere, while your doctor may do a blood test which tells exact hCG level other than just telling positive/negative in urine test.