Pregnancy test negative but pregnant?

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Is it possible to have pregnancy test negative but still end up being pregnant. We are ttc and am 20 days late this month. Did hcg urine test on 7 days late and it was negative and today did one more test ( clear blue) at 20 days late and it was negative. I normally het regular periods with 28-30 days cycle and i am very confused . Any help would be appreciated:)