I've copied and pasted this from the NHS website, and it looks as if there's not enough research to say if it's safe or not:
Herbal and green teas in pregnancy. Having said that, I work in maternity, and I'm not aware of any issues around chamomile tea
There's little information on the safety of herbal and green teas in pregnancy, so it's best to drink them in moderation.
The FSA recommends drinking no more than around four cups of herbal or green tea a day during pregnancy, and to seek advice from your GP or midwife if you are unsure about which herbal products are safe to consume. Bear in mind that green tea contains caffeine (see Caffeine).