When pregnant the calorie count goes up because you're developing a life.  Think of it like growing up.  You get taller, broader, and start to have to weigh more.  On the same token, You don't really need all that much more.  Since we all know that being overfat is dangerous, it's best to avoid more than minimal weight gain.  I can't help but laugh at those pregnancy pics of a fat woman holding her belly.  This is nothing to smile about.  Vs. a fit mom like this.