Hey-- at 5'10", a size 18 is really a size 14-16! I have a friend who is 6 feet tall and she and I wore the same size 16 pants-- but she was skinny as a rail and I was built to work in the fields (and maybe even pull the plow).
Besides, it's about being fit, not as much about the few extra pounds-- and that's medical fact (I have studies to back it up!). So go you! Be all smug! You deserve it! And heck-- it's good that the morning sickness had a silver lining to it.
Seriously, though, congratulations on having a healthy pregnancy. That's a win-win for everyone!