Can you use hand warmers while pregnant? the ones that get warm after shaking

Yes: There is no risk in using these. Best wishes for you and your baby.

Probably ok: An air-activated hand warmer contains cellulose, iron, water, activated carbon, salt, and a water reservoir. It generates heat by an exothermic reaction when the iron in the packet is exposed to air. I don't think any of these compounds will be absorbed through the skin of your hands, especially since the packet is a self-contained unit. They are not teratogens, anyways. Ask Ob-Gyn to be sure.