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.

Can you take a home pregnancy test before your period is due?