There are two major types of pregnancy tests: those that use a urine sample, and those that require a blood sample. Urine pregnancy tests may be taken at a doctor's office, or home versions can be purchased at most drugstores. All pregnancy tests work by detecting the presence of the hormone hCG, or human chorionic (kore-ee-ON-ick) gonadotropin (gone-ad-oh-TROE-pin). The body releases this substance when an egg begins to grow in the uterus. Many home tests claim to work as early as the first day your period is supposed to start. Still, when tests are taken too soon, there may not be enough hCG present, giving a false negative reading. A false positive, when the test says you're pregnant but you really aren't, is much less likely. The urine tests given by a doctor are usually more sensitive, and may detect pregnancy up to five days before your period's supposed to start. Most accurate of all are blood tests; these are available only through a health care provider. Pregnancy hormones may be picked up by blood tests just a week after conception. To confirm your test results, a doctor may perform a physical exam of the pelvic area and uterus. For more information on pregnancy tests, talk to a health care provider.