| Im a hairdresser and I agree with Layla that you should be in a well ventilated room which by code in the US every salon should be anyway.
The only thing is that when your pregnant your scalp produces more oils than normal and they are slightly differant than they were before you were pregnant, so sometimes the hormone changes in your secretions and your hair can cause the color to take a little differently. But thats only with color and toner not with bleach. Its just good to do foils or balayage instead of color so that it doesnt directly touch your scalp. But they can color in darker peices with either of those two methods to make your grow out less noticable. I would suggest balayage if you arent already doing it because it has easier growout than foils anyway. | 
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