The actual date of conception relative to your expected period, the amount of the hormone tested by the test (HCG) can vary between individuals very early in pregnancy, the concentration of your urine (fluid intake) are all variables.
While these test MIGHT detect a pregnancy at that time, a negative is less accurate. It is harder to prove a negative than a positive.
(Think of someone listening for a VERY quiet sound - if you hear the sound - you can be sure that the sound exists, however - if you don't hear the sound, it could be you are a bit hard of hearing, it could be that there is a little noise that is making hard for you to hear etc.)
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