Information gathered from another forum ...
Is it safe for me to continue dyeing my hair while being pregnant? I have heard conflicting information.
The worry here is whether the chemicals used will be absorbed through the scalp in large enough quantities and cause abnormalities in the fetus. Some animal studies have demonstrated possible teratogenic (causing birth defects) effects but there are usually disproportionately high doses of chemicals used on the animals and these amounts would never be used for the usual hair treatment. The crucial period during the pregnancy to avoid all possible unnecessary exposure to chemicals would be during the first trimester (up to 12 weeks) as this is the period of organogenesis. By the second trimester, all the organ systems are basically fully formed and the risks of abnormalities would have significantly diminished. If the pregnant woman can wait till then for the treatment, there would be less worry. The balance is between looking and therefore feeling good, and being worried about possible harmful effects for the rest of the pregnancy! Alternatively, to minimize exposure, just highlighting or streaking may reduce the chemicals used. Remember also to wear gloves and maximize ventilation should the treatment be attempted at home.