At this late stage in the game, I can't see any potential problems at all It's one of those 'no absolute evidence' ones where it's up to mum to make the choice based on her own level of concern. I avoided dyeing my hair during the first trimester JIC, but after that it was open season and I had no issues whatsoever. If your sense of smell has heightened during pregnancy due to the hormones, you may want to make sure you're in a VERY well-ventilated area as the fumes affected me more during pregnancy, but it was no biggie, just had to do my hairline in front of the bathroom mirror then move to the living room to do the rest
Enjoy having your 'do back