I have gotten my hair blonde my whole pregnancy (after 1st trimester of course) at a salon and the lady knew I was pregnant (she's been pregnant a few times too) and I'm not exactly sure if she mixed it with anything, I'm pretty sure, but I never smelled a thing and bleach is really strong smelling lol.
I bleached my hair at 25 weeks & it didn't affect the baby at all. The only thing that might happen is that it won't come out the way you want it to be. That's what happened to my hair when I bleached it lol & I always used to bleach & color my hair bright colors so it was a shock to me when it wouldn't take lol but that's pretty much it.
I heard after the 1st trimester its Fine
I would stay away from bleaching your entire head. I asked my doctor about getting high lights he told me to make sure it didn't get on my scalp
When I went to the salon to ask about that they said they like to wait until your third trimester and then they mix it with shampoo I believe so that your not getting the full fumes