A home pregnancy test measures the hormone human chorionic gonadotropin, also known as hCG. For a pregnancy test to show positive, your body must be producing a level of hCG that the test can detect. Not all pregnancy tests are the same, however. Some tests are more sensitive than others and will show a positive result earlier than tests that are less sensitive. The box that the test came in should indicate somewhere what its sensitivity is. Sensitivity is measured in IU/L, which stands for milliInternational Units per Liter. A very sensitive test may have a sensitivity of 20 IU/L, whereas a test that is not as sensitive might have a sensitivity of 50 IU/L.
A very sensitive test may be able to produce a positive results as little as 7 days after ovulation. Implantation must occur before hCG is produced, and implantation will generally take place 6-12 days after ovulation. Thus, it makes sense to try to wait at least 12 days after ovulation to test for a reliable result. If you cannot wait that long and have other symptoms of pregnancy, you might use a test that is higher in sensitivity, but keep in mind that even if the result is negative you may still be pregnant, but just not implanted.