I do not recommend tooth bleaching with pregnant women, not because there is any known medical danger, but if something happens to the baby in term, or at birth with a birth defect, then the mother will think her bleaching might have had caused it. She may not forgive herself for being vain or doing something that was totally elective. There is no medical danger that we know of (and also no research on whitening with pregnant women), but the potential psychological danger and inability to assure safety is the reason not to progress with bleaching. The baby is far more important than white teeth.
If they have started bleaching and then find out they are pregnant, I stop until birth (or six months after birth if breast feeding) to allow all the major hormones and antibodies to pass through the mother’s milk. Also, I do not want to be placing whitening products in a mother-to-be's mouth who has morning sickness or pregnancy gingivitis since it has the potential to exacerbate the problem. There is no danger in swallowing 10% carbamide peroxide when bleaching non-pregnant persons as that is used as an oral antiseptic. However, to be safe I avoid such treatment with pregnant women.