Congrats, LauraLu! How exciting! And January 7 is one of my most favorite days of the year!
I totally understand the twins fear..... and you have every right to call it a fear! It's crazy enough to anticipate the coming of ONE child, but TWO? Yeah, it's a perfectly rational and normal fear! One that if it came true, I know you could totally handle it and make it work, because you seem to be a wonderful mom, but it's a valid fear to have.
As far as being more tired and nauseated, that could just be due to the fact that you have another child to deal with.... I have found that each pregnancy has been slightly worse than the last (hence my own fears about twins!).
I have no idea what the stats are on twins and such. I was convinced that my last two kids were going to have twins, but thankfully, they didn't! Now I'm pregnant again (22 weeks), and when I first got pregnant, I had that same feeling again, but nope, it's just one kiddo in there!
I had early ultrasounds with each of my pregnancies. My first I had one because of early cramping and with my second and third, I had brown spotting and cramping.... now, with this one, my doctor just said that because of all the stress I'm under (I have three kids, we're trying to sell our house, and my husband is military and currently doesn't live at home due to training), he wanted to do an early one to make sure everything was in order.
I think you really just have to talk to your doctor and find out what his or her take is on getting an early one. Mine was all for tweaking his reasoning and calling it medically necessary to have one early on, just to get it covered with the insurance. So yours might go along with having an early one too?
Anyway.... congrats again and good luck!