There is a new trend out there to c/s fat mothers b/c fat mothers can't birth babies!!! I was the heaviest w/ my hba2c, I stopped looking at the scales at 235lbs and guess I probably put on another 10 before delivery. I was 220 and 230 w my c/s, in that order. Babie were 9.9, 8.15, 9.1.
My evidence that fat woman have no problems birthing when healthy, is that I have a very large woman who is a friend of mine and she has 4 children, all vaginal births the last one at home. And her babies are little 7lbs ers.
After Christmas I noticed I had put on more weight than I should, so I took out my WeightWatchers materials and did WW for 8 wks and did not gain a pound during that time. In the last month before birth though I packed it on big time and quickly, my urge to eat over came my ability to say I've had enough.
I did WW and became a Lifetime member between ds1 and ds2. I was asked to leave meetings when they discovered I was pregnant w ds2. The leader was nice enough to tell me that I should continue on my Maintance points until 20 weeks at which point I could then add up to 6 points as my pregnancy progressed. I failed to follow her advice, but did maintain a good weight until January w ds2.
If you have never had to count calories or watched what you eat, it can be almost impossible to do it while pregnant. You need to know how much you eat prior to your pregnancy to assess what is normal for you and then add the equivalent of a serving of dairy and a 45 calorie bread sandwich.
Good food choices in controlled portion sizes with limited refined flours, sugars, etc should give you a good place to start. Of coures, my craving foods while pregnant are donuts, brownies, cookie dough and ice cream -- all of which would be fine in moderation, but I can't eat just one brownie, I eat the entire pan in a day and a half!!!!

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Being overweight, actually catagorically obese, I believe that if you eat healthy food choices, measure your portions and count calories, points, etc it isn't what the medical profession calls 'diet'. They say 'diet' and think restrictive, fad, single food choice, not enough total nutrition etc. For me, I have to write down what I eat, measure portions, plan ahead for 'treats', and put a qualifying number next to my meals to ensure I do not over eat -- it's the only way I can maintain a healthy weight, if I fail to keep tabs, I gain weight very quickly. Maybe I'll get it right the next time I'm pregnant.
Considering the new trend to c/s fat mama's, I would make an effort to stay w/in the weight gain guidelines.