• Temporary dyes that wash out after one shampoo
• Semipermanent dyes that wash out after several washes
• Permanent/oxidation dyes that last until the hair grows out

Each preparation contains numerous ingredients that have varied over the years. For example hair dyes containing lead, used to color the hair black, are no longer in common usage except for lead acetate dyes used in Grecian Formula. In addition many formulations were changed in the late 1970s when manufacturers removed ingredients linked to the development of cancer in animals [1].

While studies on the use of hair dyes in human pregnancy are scarce several animal studies are reassuring. One study of 12 hair dye  formulations (three semipermanent and nine oxidations dyes mixed 1:1 with 6% hydrogen peroxide) found no evidence of impaired fertility or harm to the fetus after topical application in pregnant rats [2] . Another study found no developmental defects in the offspring of pregnant rats  fed five oxidative dyes at doses greater than 100 times the usual human exposure [3]. Some preparations have been associated with birth defects in mice at doses that were usually toxic to the mother [4,5].

The quantity of dye used in these animal experiments far exceeds the amount a woman would receive from routinely coloring her hair. In addition hair colorants would not be expected to be harmful with occasional use because the amount of hair dye absorbed through the skin is so small [2, 6-8].

Many human studies have asked women to look backward in their medical histories to look for an exposure  that might be related to either some birth defect or other outcome (retrospective studies). It is important to remember that the results of such retrospective studies may sometimes be influenced by the accuracy or completeness of events recalled to memory.

One retrospective study from Sweden sent questionnaires to 3706 hairdressers and 3462 women who acted as controls. The response rate was 65%. Compared with the controls, the hairdressers more often gave birth to infants that were small for gestational age (SGA), and were more likely to have an infant with a major malformation (2.8% v 2.1%). Frequent permanent waving and spraying tended to be associated with increased risk of having an SGA infant, whereas no clear association could be seen between the individual exposure assessments and malformation risk [9].

In contrast information collected prospectively from the Swedish Medical Birth Register on 12 ,046 infants born to female hairdressers failed to find an increased risk for malformations in the infants of hairdressers compared to non hairdressers. However, this prospective study did confirm that full-time hairdressers more often had intrauterine growth-restricted infants (ie, small for gestational age) as compared with nonhairdressers with a corresponding worktime [10].

The potential for the development of cancer in the offspring of women exposed to hair dyes while pregnant has also been of concern. To date studies have found no consistent evidence of an association between the risk for childhood brain tumors (CBT) and the use of hair dyes during pregnancy [11,12].

One case-control study found that the use of any hair dye in the month before and/or during pregnancy was associated with a moderately increased risk of neuroblastoma in the offspring of exposed women. Temporary hair dye use was more strongly associated with neuroblastoma than use of permanent hair dye [13]. Then again, the development of neuroblastoma has been associated with several prenatal exposures as well as folic acid deficiency [14-16].

Overall there is no definite evidence that the use of hair colorants during pregnancy is harmful.  In addition hair colorants would not be expected to be harmful with occasional use because the amount of hair dye absorbed through the skin is so small. Highlighting which doesn't touch the scalp probably poses no risk.

Nonetheless, the scientific evidence is too limited to give to draw firm conclusions for or against the use of hair colorants during pregnancy. There are no studies to suggest that “natural” products are safer than man-made ones.

Any potential for harm may be reduced by taking the following precautions:

• Avoid the use of hair dyes during the first trimester.
• Use protective gloves.
• Work in a well ventilated area.
• Do not leave the dye on the hair any longer than necessary.
• Rinse the scalp thoroughly after the use of hair dye.
• Reduce the frequency of hair dying.

REFERENCES
1. Paul , Mauren (ed.) (1993). Occupational and Environmental Reproductive Hazards: A Guide for Clinicians Baltimore: Williams and Wilkins 366-367.
2. Burnett C, et al. Teratology and percutaneous toxicity studies on hair dyes.J Toxicol Environ Health. 1976;1(6):1027-40. PMID: 966314
3. DiNardo JC, et al. Teratological assessment of five oxidative hair dyes in the rat.Toxicol Appl Pharmacol. 1985 Mar 30;78(1):163-6. PMID: 4035668
4. Inouye M and Murakami U.Teratogenicity of 2,5-diaminotoluene, a hair-dye constituent, in mice.Food Cosmet Toxicol. 1977 ;15(5):447-51. PMID: 598797
5. Marks ta, et al. Teratogenic evaluation of 2-nitro-p-phenylenediamine, 4-nitro-o-phenylenediamine, and 2,5-toluenediamine sulfate in the mouse.Teratology. 1981;24(3):253-65. PMID: 7330774
6.Wolfram LJ et al, Percutaneous penetration of hair dyes.Arch Dermatol Res. 1985;277:235-41. PMID: 4015186
7.Maibach HI, et al. Percutaneous penetration following use of hair dyes.Arch Dermatol. 1975;111(11):1444-5.  PMID: 1200652
8. Bronaugh RL and Congdon ER.Percutaneous absorption of hair dyes: correlation with partition coefficients. J Invest Dermatol. 1984 ;83(2):124-7. PMID: 6470515
9. Rylander L, Reproductive outcome among female hairdressers.Occup Environ Med. 2002;59(8):517-22. PMID: 12151607
10.Rylander L and Källén B.Reproductive outcomes among hairdressers.Scand J Work Environ Health. 2005;31(3):212-7. PMID: 15999573
11.Holly EA et al. West Coast study of childhood brain tumours and maternal use of hair-colouring products.Paediatr Perinat Epidemiol. 2002;16(3):226-35. PMID: 12123435
12. Efird JT, et al, Beauty product-related exposures and childhood brain tumors in seven countries: results from the SEARCH International Brain Tumor Study.J Neurooncol. 2005;72(2):133-47. PMID: 15925993
13. McCall EE, et al. Maternal hair dye use and risk of neuroblastoma in offspring. Cancer Causes Control. 2005 Aug;16(6):743-8. PMID: 16049813
14.Cook MN, et al. Maternal medication use and neuroblastoma in offspring.Am J Epidemiol. 2004 Apr 15;159(8):721-31. PMID: 15051581
15. Bkuhm EC et al., Maternal use of recreational drugs and neuroblastoma in offspring: a report from the Children's Oncology Group (United States).Cancer Causes Control. 2006;17(5):663-9. PMID: 16633913
16. French AE ,et al., Folic acid food fortification is associated with a decline in neuroblastoma.Clin Pharmacol Ther.2003;74(3):288-94. PMID: 12966372