Ummmmm....hate to say it, but you might just not be eating enough. What's your caloric intake/nutrient load/sheer volume?
We all eat more when we're pregnant, because low and behold, we're growing a whole 'nother body inside of us. Yikes. Way to make pregnancy sound yucky.
Anyhow, it may well be possible that you're pooping less not because you're constipated, because your body is using every last bit that it can squeeze out of what you're eating. <grinning> While you may be able to justify adding a twinkie, try to bulk up in proteins (the most... 100 grams of protein a day is what's recommended by many OBs), and rich leafy greens.
And pay attention to those cravings. Craving icecream 24/7? Try spinach (mmmmm Saag Paneer), whole milk, yogurt, or other calcium rich foods. Follow your cravings (they tend to indicate what your body needs at that given time), but to try and examine them. A craving tends to follow the line of something we eat regularly or know well. If you can isolate what the craving is actually for...you can vary your diet in a healthier way. Burger cravings can usually be satisfied by lox, or cannellini beans on crunchy italian bread.
Eating more can be hard for some of us. I'm anorexic (not that you'd know it by looking at me these days), and actually had to quadruple my caloric intake. Ugh. I persoanally went the mental route of "I can screw around once my son is born, but until then, this is about him...not me". And on a flatline metabolism I doubled my weight. And lost over a hundred pounds the year he was born without even trying (aka, still eating). Other's are so used to dieting, that the concept of eating more doesn't even occur to them. (Not we anorexics, food is always on our minds!) Still other's don't really have the money to spend on extra food. And yet still others are so afraid of gaining weight while pregnant, that they start eating less then they did when they weren't pregnant! Doh. Wrong option.
Anyhow...our elimination schedules frequently change when we're pregnant, but I thought i'd toss this possibility out there with the others.
:) Good Luck