I've read up a lot on this as my hair needs doing badly. Basically what I found was that it's best to avoid before 12 weeks but thought to be OK after that. I have met a number of women IRL who have had disasters with home hair color when pregnant (getting totally different colors to what they normally get with the same product) so I'd leave it to the experts.

I found some articles that said that very little research has actually been done on hair dye safety when pregnant so it's really not known if it has an effect, it's just thought not to. Foils are thought to be safer as they don't touch your scalp.

I'm not taking any risks with this one so I've been living with my greys but am thinking about getting some foils done in a few weeks.