It's not known whether the chemicals used in hair dyeing, perming or straightening are completely safe for you to use during pregnancy. One study suggested that it could cause the foetus to develop the neuroblastoma, which is a malignant tumour of the embryonic nerve cells, but there have been no other studies to verify this. It may be possible that a few of the chemical compounds found hair dyes could cause birth defects, but they would have to be used in extremely high quantities, far greater than the amount you would use over the course of a year. If you apply the dyes safely, colouring your hair up to three or four times during pregnancy is probably safe.
Follow the instructions carefully and apply the product for the minimum length of time. Wear gloves and work in a well ventilated room. Look for a dye with little or no ammonia or peroxide. Instead of dyeing your hair you may want to highlight, paint, or frost it. Hair colouring agents are absorbed through the skin on the scalp, not the hair shafts. By having highlights or streaks means your skin has less contact with chemicals.
Vegetable or plant dyes may be a good alternative, but check the packets. Some vegetable dyes may contain the same chemical compounds that are found in man-made dyes. A safer bet would be to use pure henna. It is a vegetable dye that's very safe to use, is semi-permanent and comes in a number of shades.

Little is known about the affects of using chemicals to perm or straighten your hair during pregnancy. One particular study showed that using these products would not increase your risk of premature birth or having a low birth weight baby, but it didn’t mention birth defects. Only a very small amount of these products will soak into your scalp during a treatment. If you decide to have your hair permed or straightened, perhaps only have one treatment, to limit your exposure.

You should feel good about yourself while you are pregnant. So consider whether colouring, perming or straightening your hair will make you feel good or just make you worry. Just to err on the side of caution, many expectant mums-to-be wait until their second trimester to have these beauty treatments.