Home pregnancy tests use a sample of a woman's urine to detect the presence of a hormone (called human chorionic gonadotropin, or hCG) that is produced during pregnancy. If the test is positive for the hormone, it usually means the woman is pregnant.
These tests are available without a prescription, and their accuracy varies. If used correctly, the best tests can detect hCG as early as the first day of a missed menstrual period (about 14 days after conception). A first morning urine sample (which has accumulated in the bladder overnight) provides the most accurate test results.
WebMD Medical Reference from Healthwise