Green tea has some caffeine in it, so if you're avoiding that I guess it would make sense. The evidence is that caffeine is fine, unless you're really overdoing it.
A quick google did find this
http://www.buzzle.com/editorials/1-24-2006-87222.asp, which explains that green tea molecules have a similar structure to methotrexate, which is used to induce abortion. But really--you'd need to be drinking a crazy amount of green tea to have any effect.
Chamomile is a mild uterine stimulant, so one should avoid the concentrated versions (oils etc). But an occasional cup of tea...I wouldn't worry about it.
If you have a history of miscarriages, and are early in your pregnancy, you might avoid these as you avoid other stuff. But later on, in a healthy pregnancy, I'd feel comfortable having both of these on occasion.