Negative ions are found abundantly in nature, particularly next to running water. They are odourless, and completely harmless. Please see the following link for more information on negative ions:
http://www.webmd.com/balance/features/negative-ions-create-positive-vibes
The natural substance that is used to generate the negative ion is tourmaline crystals, similarly there are no health warnings relating to pregnancy. Negative ions are beneficial throughout pregnancy and beyond.