NO IT IS NOT SAFE to change the litter box when pregnant, unless you are properly protected. Always wear long rubber gloves and a mask. Be as careful of inhalation of the dust particles as contact with your skin.
You might want to try cat box liners, as these are much cleaner, no mess and keep you from touching the contents of the litter. Always wear a mask, pregnant or not as the dust particles can contain harmful bacteria as well as the litter itself.