I have read that in early pregnancy it can be bad and cause you to have a miscarriage and that you should have it only until you ovulate and then are safe to have it to bring on labour at the end of your pregnancy
Agree with PPs that it's not advised to drink in early pregnancy. In the last trimester its recommended to improve or ease vaginal delivery - don't know the technical bits of why though. I could only stomach one cup a day but a friend of mine was having up to 5 (!) cups a day towards full term. I got mine in Holland & Barrett, try a local health or vitamins shop.