it's safe and as mentioned above, the hormones may take the color differently. i have had mine highlighted twice since i have been pregnant and it took to my hair the same as always. i did wait till i was out of first trimester just because i was too paranoid to do anything first trimester. plus, i thru up every day so i figured i'd end up embarassing myself if i yacked at the salon!
One of my pregnancy books said it's fine but if you want to be totally safe/conservative (which i did!) to wait until after the first trimester. I actually think it was nice to give my hair a break anyway:)
When I asked my OBGYN when I was 8 1/2 weeks if it was ok to highlight hair, he said that was fine it was coloring your whole head (as in dying it) is the issue because the color seeps into your scalp. He said if anything I should maybe wait a few more weeks after the first trimester but it was definitely ok (since it isn't going into the scalp). I just got my hair re-highlighted last week and it feels oh so good!
My RE recommenced no color / highlights in the first trimester. I was allowed after that. My regular gyn isn't so strict.. so she said it was okie ...
It is safe, but your color may come out really off due to hormones.. Like if you are getting blonde highlites, they may come out orange... =P
Good luck
I asked my doctor that when we had our first appt., and he said it is fine. He said to do it maybe half as often. My friend who does my hair said she has highlighted the hair of many pregnant women.