Hair dyes are generally thought to be safe to use while pregnant, according to American Pregnancy and the Mayo Clinic. Some of the chemicals can penetrate the skin, but the amount is most likely too low to affect the developing fetus.
Erring on the side of caution is still wise, advises What To Expect, which recommends waiting until after the first trimester, when the baby is developing its organs, muscles and vocal chords, to use hair dyes. Always follow package directions, including wearing gloves and using hair dyes in a well-ventilated area. Leave the dye on only as long as the package directs, and rinse the scalp thoroughly with plenty of water, advises the Mayo Clinic.