If you have excess weight and eat healthfully/exercise during your pregnancy, you could lose weight while being pregnant. That is hard to do because sometimes being pregnant makes you feel hungry almost constantly. Of course the best time to lose weight is before or after a pregancy. Don't be too hard on yourself if you don't feel like losing weight while being pregnant. But if you want to undertake it, I have some ideas. Personally, If it were me, I would go forward with the diet plan you were going to do, but add in extra servings of fruit, vegetables, whole grain breads, milk, low sugar/fat yogurt as needed. It seems that Weight Watchers may have a program for nursing moms which would probably be good for pregnant women as well.
Additionally, here are some ideas and tips to consider:
1) Research shows that people who regularly eat breakfast weigh less than those who don't.
2) Research shows that women who drink milk regularly weigh less than those who don't.
3) Research shows that women who get 8 hours of sleep weigh less than those who don't get enough sleep, this especially applies to post-pregnancy (making a great case for new moms to nap when their babies nap, the ideal time being 12 hours after your night-time sleep midpoint).
4) Pregnant moms only need an extra 200-300 calories per day (a piece of toast with a little butter and a banana for example). So it's easy to gain more than the recommended 25-35 pounds... too easy since many of us feel like eating all the time while pregnant... add emotional eating or snack popping while watching tv into the mix and no wonder we are gaining too much.
5) Take inspiration from people who lose weight successfully the old fashioned way (watch the Biggest Loser, or at least the finale).
Lastly, I will say that it is definitely lifestyle changes and health habits that make for long term good health and healthy weight. I lost 60 pounds, but it took me 6 years to do... little things like not eating in the eveing after dinner when my body didn't need food, not wasting calories on things that I didn't love (for instance, to me doughnuts are just okay, so if I have the opportunity to eat a doughtnut, I don't), avoiding buying things that are a temptation, eating smaller portions but really savoring what I like.
Recently my weight was creeping up and I went off sweets for 3 weeks to get myself out of the habit of snacking.
I guess I have said enough and I hope something I said is helpful. Good luck H.!
Also, Walking and swimming are great exercises that are not too hard on a pregnant body.
One last thought, we are all different and our bodies are different... things like heredity and genetics have a lot to do with body type. Focus on healthy habits and loving yourself and your body. God loves you and He wants you to love the beautiful you He created! You do that by doing your best, then accepting yourself, imperfections and all!