How did you go at the GP? When I started reading your post I was going to suggest going to the GP for a blood test to check for certain - they could perhaps to an initial blood test to check your levels, then do another test 2-3 days later to see if those levels are rising (doubling) to see whats going on for sure.
I've had a m/c at 5-6 weeks and I got faint BFP's but then brown spotting began followed by red flow so I kind of knew straight away without having to wait, even though I continued to get faint BFP's for a week after the initial bleeding began. However I had a friend who had a "missed miscarriage" and it was only detected around 8 weeks, then she had to have a D&C. So everyone is different really. I'm sorry I can't give you any other positive stories, however I have definitely read stories on various forums where some ladies continued to get BFN's even though AF never arrived and they definitely turned out to be pregnant and carried bubs to full term! So there is always hope!
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