Hey, Congratulations on your pregnancy and your engagement!
OH didn't propose to me until after LO was born, so I personally didn't plan whilst pregnant but I think it would be a fab time to do it.
You have plenty of time on your hands, you can get virtually everything sorted, and when your little one arrives you don't have to worry about putting the wedding plans on hold because they are already done.
I sometimes struggle to get wedding things done because a lot of it is website based searching for things (tiara, wedding dress hoop, etc etc) and trying to juggle Amelia, work, housework, exercise, spending time with OH and wedding planning is very difficult!
Good luck and I hope your enjoy every second of your planning!! I find it so much fun (when I have the time haha)
xXx