ok so i'm now 3 days late n still no af symptoms but still Bfn

does anyone know the longest it can take for hcg to show in urine??
i know i'm clutching at (very small) straws but oh still think's im preg
aaaaaaaarrrrrrrrrrrggggggggggggggggghhhhhhhhhhhh!!!!
this is driving me crazy!!!!!!!!
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