After the first trimester and do it in a salon or outside where you can have good air flow. The fumes are what's harmful.
I would suggest dying it a color that u won't have to upkeep so much just to be on the safe side of you not having to introduce the chemicals over and over again.
O and my doc and MW both said don't get it on your scalp! That way you are taking every precaution. So maybe a friend or someone can take the time to ensure they don't get it on your scalp my hairdresser did it as close as she could without touching my scalp and you couldn't see my roots (:
Usually anytime after the first trimester is fine! I got highlights when I was 38 weeks!
i asked my doc if i could dye my hair and she told me its fine and doesn't harm the baby. that was when i was 4 months.
I heard and seen my sister dye her hair when she was almost 9 months. And my hairdresser said its fine to dye your hair when ur 8 months. Ask the doctor as well to be on the safe side .