Hi Mommy! According to what I read , the reason as to why pregnant women are not allowed to dye their hair is that because you might smell the strong scent of the chemicals and you as a pregnant woman who has a very sensitive sense of smell, might feel nausea and may lead to contraction which leads to miscarriage. Technically it's not because the chemicals will go through your scalp, it's because of the strong scent that it has.