As long as you eat healthy, I wouldn't even follow the guidelines. My gyne told me that they are lowering the figure from 25-35 pound weight gain to 15-25 pound weight gain!! It's not realistic. I put on a wopping 60 pounds by the time I was 9 months pregnant and I'm pretty sure I put on another 5 pounds in my final week.
I wouldn't follow the guideline only cause I've noticed that the taller you are, the more weight you put on. Quite a few girls in my height range put on 50+ pounds, but that was normal for their body/height which is why it came off.
I lost 50 pounds within 6 months and the last 10 are slowly coming off. Eat healthy and walk everyday and you'll be fine.
If you walk a lot during pregnancy, does it help somehow to lose the weight after baby is born? My 1st trimester I lost about 10 lbs. And once morning sickness went away, it seemed I gained weight overnight, but mostly in my belly. I still have about 2 1/2 months to go! I try to eat balanced meals, but have my binge days where I go nuts with cookies/cake etc.