Heating Pads

I've been using a heating pad frequently since about 4 weeks pregnant, now 10 weeks. I've been putting it on my abdomen (wrapped in a towel) and thought that was fine, because my doctor said heating pads were fine for cramps. I've now read several articles on temperature increase and neural defects. Did anyone else do this and were they fine? I'm not going to do anymore, but freaking out a bit!