Avoid henna and ammonia because they are absorbed through the scalp more readily and the effects on the fetus are unknown...but with that said, most doctors find it perfectly safe to dye your hair after the first trimester and I've done it with 2 out of my 3 pregnancies..it's a great pick-me-up for the middle-of-pregnancy blahs!
and yes, a well-ventilated area is a MUST and often you get best results when you let a professional do it because of the way our hair changes during pregnancy :-)
Oh wow I hadn't even considered that it would make me sick...I haven't gotten sick once since I've been pregnant ( im16 weeks)...I may hold off a while longer
My doctor said all hair dye is okay, they just prefer you use it in the second trimester. Ammonia wont hurt the baby, but it can make you nauseous.
make sure your in a well ventilated area, even if your product doesnt contain amonia and what not it may make you feel sick just from the strong smell itself!
as long as you use a natural colour .. or a colour with no peroxide and amonia your fine .. oh and as long as you wait until after 1st trimester!.. but semi perminent colours are fine =]