DDCC -
because, from your situation, I would have to consider that ovulation happened between the 21st and 25th... and with the stats already posted (which I very much agree with, once you have THAT high of hcg, it does not double very fast at all, if at all) I would say... take a DEEP breath and a warm bath and try to relax some.
putting together your timing for DTD plus your timing of the bfp (a faint positive most likely happened between 9 and 13dpo... while you can get faint ones outside of that range, that is the most common time for them to happen) shows that likely everything is adding up just fine.
Many women end up seeing an empty sac even up and into their 6th week (with DD my nearly 7 week ultrasound just barely showed heartbeat...) if you were just over 6 weeks at that ultrasound, it would make you just over 7 weeks now... which lines up with those dates.

and praying that all is okay for you... but really I have a good feeling it is!
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