Wow, congrats OP! Glad the baby looks healthy and how lucky you were taking the vitamins already!
In my first pregnancy, I didn't start to show until 25 weeks, didn't go into maternity clothes until I think 28 or 30 weeks (and I really didn't need it yet, I was just anxious), but the belly came really fast after that. I went to my anatomy scan w my regular jeans on, buttoned and everything. I had really bad ms, so I ended up finding out at just before 10 weeks because I knew something was us, but my periods are irregular too, so skipping a month wasn't usuaual. If not for the ms, I can imagine that I'd have gone quite a while without knowing.
My friend took a ton of at home tests and they were all negative, but she didn't have her period for months so she went to the dr finally and she was somewhere around 13 weeks along. This was her second, so she even already knew what pregnancy 'felt' like. Her baby came two weeks early, perfectly healthy, so she might've been further along than they though. They dated it strictly by u/s measurements because it had been almost six months from her last lmp (apparently, she skipped a few months, then it started back up and she ovulated and it 'caught' so the rest of the 'skipped' months were because she was actually pregnant.
I'll be coming back in a few days to see how your big scan goes!