I actually slimmed down during preganancy, coz I only felt like eating fruits and veges and yoghurt.....couldn't stand the site of a chip!
(I stacked it on when I had two toddlers and couldn't find time to exercise, and couldn't stand seeing food go to waste!)
I agree with your Doc. I would say concentrate more on getting fit and healthy. Pregnancy and birth put alot of strain on your body, and you'll fair much better if you are as fit as you can be.
Also, belly dancing is a fantastic way to condition your pelvic muscles.
You also lose weight while breastfeeding.