According to WebMD, very little, if any, chemicals in hair dye that are absorbed by the body reach an unborn baby. Dying hair is likely safe during pregnancy as long as certain precautions are taken, according to the Mayo Clinic, such as following package directions, leaving the dye on for the minimal amount of time directed, using gloves when applying the product and rinsing the scalp thoroughly after dying hair.
WebMD advises not mixing different dyes, not dying eyelashes or eyebrows and performing a patch test before using the dye. What to Expect states that waiting until the second trimester of pregnancy may cut back on any slight risks possibly associated with any absorption of hair dye through the skin, because the baby will have grown beyond important developmental milestones, such as organs, muscles and vocal cords that form during the first trimester. Parents Magazine states that it's even safer to wait until the third trimester, when most of the baby's major organs have finished forming. What to Expect also advises using hair dye with plenty of ventilation, using the gentlest dyes possible and opting for highlights, lowlights, frosting or streaking rather than applying dye to the roots, where the dye comes into contact with the skin. Pulling hair through a cap for application is also a safer way to use dye.