I was told second trimester in a well ventilated area.
Its safe my midwife told me that dying my hair is fine just as long as its every 4-6 months in between dying your hair
It is safe I just asked my physician about this Thursday and he stated its OK in the second trimester... I actually just did my hair and I used a demi instead of permanent it has no smell or ammonia in it. I won't do it all the time but just to have my hair decent until my bundle of joy arrives ;)
I'd be more worried about breathing the fumes than the dye actually being on your hair. Could stick to semipermanent. I know many women who dyed while pregnant and the kids are all fine. I have dyed, and I am normally pretty strict about being careful and healthy. I think the risk is little to none. Don't let the dye sit all over your skin and try not to breathe it in, make sure you have ventilation. That all said I am not a doctor and this isn't medical advice, just opinion.