Congratulations, and first of all, stop worrying
! Enjoy and embrace this time, even if you do feel like poo, it's one of the best things you can do for yourself.
You need to do what makes yourself comfortable, but there have been many a successful pregnancy completed while on a LC WOE, my own included. I remained VLC for most of my pregnancy after doings loads and loads of research. My pregnancy was amazingly easy, I didn't even care that I went past 42 weeks, I wasn't uncomfortable in the least. Recovering from labor was not bad and my son has always been incredibly healthy (he's 2y 8mos).
I also had bad constipation during pregnancy, I ended up taking colace and a couple tablespoons of liquid magnesium citrate every morning somewhere around the end of the first trimester. I was also exhausted the whole first trimester... it's hard work making a placenta!! I never had any morning sickness though, I attribute that to LC.
On contemplating changing back to your old "non-low carb lifestyle"... if it was not healthy for you before, do you think it will create a healthy environment in your body to grow a baby?
Unfortunately pregnancy losses occur for so many reasons and most of the time we never know what was the cause. I can only imagine the trepidation you have with carrying this pregnancy after suffering a loss. Early pregnancy symptoms can come and go... don't over analyze or read to much internet stuff, get lots of sleep and naps and drink plenty enough water.
I gotta run, I hope this was helpful, have a great day