There is apparently a lot of debate in the ob/gyn community about how caffeine consumption affects pregnancy. The American College of Obstetrics and Gynecology (ACOG) just released an opinion stating that <200mg (about one cup of coffee) per day does not increase the risk for miscarriage or preterm birth. No conclusion could be made about the relationship between caffeine and intrauterine growth.
Dr. Barth, Chair of the Committee on Obstetric Practice, was quoted as saying, "Given the evidence, we should reassure our pregnant patients and let them know that it's OK to have a cup of coffee."
I'm not too surprised about ACOG's opinion, given the amount of tea South Asians drink, and I can't imagine all of our women giving up chai during pregnancy! Tea also has less caffeine content, about 100mg, than coffee, which may help. A reminder to pick your addictions wisely! :-)
Of course, one should always make any decisions regarding pregnancy and caffeine intake with one's physician, as every situation is different and depends on any underlying medical conditions.