The expert answers: Home pregnancy tests work by detecting the level of a hormone, known as human chorionic gonadotropin (hCG). Levels of this hormone being to increase in the body immediately after fertilization. This hormone usually appears in the urine three to five days after conception, but is not normally detectable for 10 to 14 days. hCG peaks at about two months after conception and then gradually decreases as pregnancy continues.
Home pregnancy tests should be used after the first missed period for best results. Testing too early is not advised, since it can cause false negatives, giving you the false impression that you are not pregnant when you really might be.
False positives may also occur if you are using drugs which contain hCG, such as Humegon or if have certain rare medical conditions.
Make sure you read the package directions carefully and follow them exactly. When used correctly, these tests are 95% accurate. If you doubt a test result, we recommend testing again or contacting your doctor, who can perform a more sensitive blood test to confirm your results.