I was told by my cousin who is a hairdresser it's fine s long as your in a well ventilated area. It's not the chemicals against your head that are harmful it's breathing it in that can do the damage.
Here's some advice on what to do about toxins from MedHelp.org and what's safe in pregnancy -- specifically the slide on going to the salon!
http://www.medhelp.org/pregnancy-health/slideshows/Is-It-Safe-Top-Pregnancy-Questions-Answered/2158/6
https://
www.medhelp.org/pregnancy-health/articles/Chemicals-and-Other-Toxins-to-Avoid-During-Pregnancy/2086
You can also find it in the Week by Week section of the I'm Expecting app -- check Weeks 7 and 13.
Like special says, you will do OK in terms of safety, but sometimes pregnancy affects your hair in ways you don't expect, so you might get different results than you anticipate, especially if by "bleach" you don't mean gentle lines here and there but actually bringing it to something very light. I'm not pregnant but presently growing out some pretty radical highlights, and remind myself of Miley Cyrus with her dark roots and blonde bottom half of the hair, and it is driving me crazy. When I shampoo, I have to be careful to ultra-super-condition the bleached-out part or it feels like dead broomstraw, but not let the conditioner hit my roots (which is hard because by now they are 2 inches long) or else my non-damaged hair will be too limp and my hair will just lie plastered to my scalp.) In short, do a sample patch if big bleaching is what you're after. :)
I think today's products are pretty safe. Well ventilated rooms help. I had my hair highlighted throughout my pregnancies and also had a base color at times applied as well. I mean, you have got to have good hair when pregnant, right? So, I would run it by your doctor but you should be good to go with your hair. good luck