While there are very few studies of hair dye use in human pregnancy, animal studies have shown no adverse effects. Many women have dyed their hair during pregnancy with no reported harmful outcomes. Only a minimal amount of hair dye is absorbed through the scalp after application and this small amount is unlikely to harm your baby. If you dye your hair with a single process and you’d like to be especially cautious, wait until after the first trimester to dye your hair. Highlights should be fine at any time since hair dye is not applied directly to the scalp.