The heat pack is for your lower back I think. Like using a hot water bottle when you have period pain. Not sure about the baby. The hospital I'm going to doesn't microwave the wheat bags anymore (someone managed to get one to explode), so I need to take one of the gel packs with me (you can put it in the freezer or warm/hot water). You may want to check this wherever you are going too. I don't use water bottles when I have bad period pain, but I have found that heat on the bottom of my back helps at the moment when its hurting.
As for over packing, I keep reading the lists, then rereading, thinking "am I really going to be needing all this stuff?" Surfboards, okay, but I'm only going away for 2 nights! Then the Man pointed out that it could be longer. I suppose, and I don't want to have to get him to go home after work to pick something up for me on his way to see me.