According to currently available data, First Response tests are the most sensitive and can therefore detect pregnancy the earliest. However, if you have a reason to be concerned about your pregnancy—for instance if you have a history of miscarriage—you should talk to your healthcare provider and they may have you take a serum, or blood pregnancy test. In general, these tests can detect pregnancy earlier than over-the-counter urine tests. And because they measure the amount of hCG, as opposed to its presence, they can give your doctor information about whether your pregnancy might be abnormal or even dangerous due to situations like early miscarriage or an ectopic pregnancy, which could be associated with unusual patterns in hCG levels.