At-Home Pregnancy Tests. The levels of hCG rise rapidly after a woman becomes pregnant, but there needs to be a certain amount in the body before a test can pick it up.
Blood tests are the most accurate and sensitive, while urine tests take a little longer.
Although the instructions on most home pregnancy tests will say that you need to wait until two weeks after a missed period, the reliability of any pregnancy test has nothing to do with whether or not you have missed your period. The only factor is how long a woman has been pregnant. The urine tests used by a doctor are more sensitive and can pick up a pregnancy one to two weeks following conception. However, some new forms of at-home pregnancy tests are starting to hit the market that are almost as effective as the kind previously available only to doctors. They are said to be effective within a day following a missed period.