During the early stages of pregnancy, your body will produce more hCG. As the weeks pass, your body will produce more and more hCG, increasing the likelihood that a pregnancy test will be positive. Pregnancy tests operate by detecting the presence of hCG in your urine or blood. Most tests also have lines on the card where you place your sample; these lines indicate whether or not your test was positive for hCG. When you think about it, your body produces more hCG when you're pregnant, so it shouldn't come as a surprise that there are substances out there that can detect hCG. There are two types of pregnancy tests: those that use your urine and those that use your blood. Both types work by detecting the presence of hCG in your body fluids. The only difference is how they do it. Urine tests rely on antibodies to hCG protein to identify its presence in your urine, while blood tests use enzymes found in hCG but not in other proteins found in blood. Blood tests are more accurate because they don't depend on what type of specimen you provide. Pregnant women's bodies produce large amounts of hCG, which is why most pregnancy tests become positive around day 14 after conception. But because hCG is also made by the placenta and unborn baby, some women find that their tests remain positive long after they've taken all possible pregnancies out of their system.