You can dye your hair before the third trimester. I was in hair school for the first 18 weeks of my pregnancy and I worked with more harsh things than hair dye... Such as perm and relaxer solution. I wouldn't have been able to continue in school if it was unsafe. However, I would suggest being in a well ventilated area... I just open the window in my bathroom.. Or if you don't have a window in your bathroom, turn your fan on and leave the door open.