Use hair dye that has no ammonia. And even with no ammonia try not to breathe the fumes, trying putting something over mouth and nose. But I dyed mine at 19 weeks but I used no ammonia. And I went darker cuz I had tri colored hair.
Every woman is different so for some the hair won't take the dye but for others it will. I've dyed my hair throughout both of mine and never had any problems with baby or my dr telling me not to, and I didn't just make subtle changes I went from lighter to darker to lighter again. There aren't enough chemicals in the dye that seep into the scalp and get into your system to cause concerns. It's mainly the smell of the processing chemicals (ammonia) that is bad for you. As long as where you get your color done is nice and ventilated you should be fine. If you go to a professional, some of them even have special dyes without the harsh ammonia smell that might be a better option for you. You can also highlight/lowlight, less dye gets on your scalp when they pull your hair through a cap. Hope this helps!
It depends on the person as far as hormones go. And my doctor told me wait til 16 weeks to dye hair. With my first my hair would take color no problem, with my second you couldnt even tell i dyed my hair, with this one i dyed my hair on july 4th @ 2 weeks pregnant (because i didnt know) and it took well but it coulda been because i wasnt that far along. I would do a test dye on a couple strands in the middle of yout head that wont be seen, see how it goes and make a decision from there.