I have dont have an answer on getting it done while pregnant i think its up to you.
Now as for the colour. I too went blonde from dark brown and it took me a long time. My best friend is the manager of a rather top salon, and she did this for me at home I only ever did a whole head of highlights, i think this is a better option than doing the whole head of hair. The other thing to think about and i know this sounds contradictory is if you put 2x blonde highlight and then 1x light brown it actually makes you look more blonde ( i know it sounds crazy i was really shocked). And it also stagers your re growth so there is not one solid line when it grows out.
If there is a particular salon that you like, you could ring them up and ask them if they have trainee's that need to practise colouring, and that way you may even get it free.
I do also agree with it changing your skin tone! I was shocked when i first went blond ( I had thought about it for a long time).
good luck hope it makes you feel fantastic!!!