I found this great article (maybe from Parents magazine maybe?) about what the extra weight actually is when you're getting ready to deliver:
Breast Tissue =
1-2 lbs
Baby =
6-8 lbs
Uterus =
2 lbs
Placenta =
1.5 lbs
Amniotic Fluid =
2 lbs
Blood Volume =
4 lbs
Stored Fat =
4 lbs
Fluid Retention =
4 lbs
Not one mention of Nutella or kettle cooked potato chips. See, I told you.