Pregnancy tests do not typically work after one week of pregnancy, reports Bounty. Pregnancy tests usually work after two weeks of pregnancy because that is when tests can detect human chorionic gonadotrophin, which is a hormone that increases quickly during pregnancy, in a woman's urine.
This two-week delay often coincides with the time that pregnant women miss their periods, creating a convenient time to test, notes BabyCentre. Pregnancy tests vary by sensitivity, which affects how soon they can accurately detect pregnancy. These sensitivities range from 10 milli-international units per milliliter to 40 milli-international units per milliliter, with the lower numbers indicating higher sensitivity.