Why do some pregnancy tests look negative after a few days

[Mamastarz]

Basically what the title says.
Why does this happen, not all tests, but some. Positive within less than the time frame, but 2 days later negative.

I'd imagine it's because sunlight on them breaks down the dye so they fade bit like anything that's left out really

I've still got tests from DD1 though and can still see the lines 6 years later

I find blue dye ones (imo) are the worst for it. But i see what your saying, its sounds about right.

Really?

I have my FRER from DS2 and its still very positive 2.5 years on!