I've always waited until 2nd Tri. I'm not sure on the reasoning. But I bleach and use semis.
A professional won't do anything different to your hair - and as long as you're using 20vol peroxide you are pretty safe. Stay ventilated. And yes, lightening OR darkening can produce slightly different shades, but most people are fine. My new hair did go a slight different shade while pregnant, but it was nothing obvious.
ETA: Looking at the product you linked, you should be absolutley fine.
What I would however suggest for every pregnant lady, is to do a skin test 48hrs prior. Your hormones can cause a reaction to products you've always been fine with. I would certainly do it with high lifting shades like that one