In pregnancy, the first trimester is crucial as it's the period of major organ development. Some chemicals in hair dyes, particularly dark colors, may be linked to birth defects or pregnancy complications. 

Hair dyes contain chemicals that may be absorbed through the scalp and enter the bloodstream. The skin's increased sensitivity and altered absorption rates during pregnancy raise concerns about allergic reactions or irritation. Ammonia-free and PPD-free dyes are recommended as safer alternatives for hair colour during pregnancy. Highlighting techniques that avoid scalp contact can reduce exposure. 

Applying hair colour during pregnancy, ensure good ventilation during application to minimize fume inhalation. Consult healthcare providers to make an informed decision balancing personal preferences with potential risks.