I don't think there is anything on the market that in the dose of a cup of tea a day could cause any harm (although I do tell my patients to try to avoid herbs in general during the first trimester when the fetus may be particularly vulnerable).
Teas are so much more diluted than herbal tinctures, for example. I would avoid teas that contain ingredients with well-known pharmacological actions, such as kava or St. John's Wort.
I would also stay away from teas that might cause the onset of menstrual or uterine contractions, such as black or blue cohosh.
Raspberry leaf tea is commonly used as the foundation of almost all pregnancy teas and is completely safe. It's thought to help promote effective uterine contractions at delivery.
Another safe nutritive tea is Aveena or oat straw tea, which is beneficial for the nervous system if you're anxious or depressed.
Nettle tea is also safe in pregnancy and it contains a lot of nutritive minerals.
Vegan diet during pregnancy
Chocolate during pregnancy
Unpasteurized juices during pregnancy
Tap water during pregnancy
Drinking milk from cows that were given BST during pregnancy
Caffeinated sodas during pregnancy
Bleaching teeth during pregnancy
Vacation at high altitude during pregnancy
Swimming in a chlorinated pool during pregnancy
Riding the bumper cars at an amusement park during pregnancy