I dyed my hair during my second trimester after my gynae said it was fine, just make sure the salon has good ventilation and you're not doing it too frequently, lah. Most doctors say occasional dyeing is okay once you're past the first trimester, but I was honestly a bit paranoid about it anyway so I waited until I was further along. Best thing is to just check with your OB/GYN first, they know your situation best and can give you the clearest answer for your own pregnancy.
It’s safe!