You may want to also post this question under the ask the expert section to get a response from the MD.
I have a friend that had a uterine fibroid that was only found after she found out she was pregnant. I don't know how large it was but I do know that in the initial months it grew while the pregnancy hormones were "feeding" it, but eventually this subsided and it shrunk somewhat. She had a scheduled c-section but I believe it was blocking the way out for the baby, so as Juana said, it may depend on the size and location of the fibroid near the end of the third trimester.
Best wishes!
Congrats on your pregnancy. It really depends on where your fibroids are located. Your doctor should be able to tell you as you get closer to your due date, if the fibroid will pose a problem. I had fibroids during my pregnancy and would have delivered normally, if my blood pressure would have not went up so high. Best of luck to you!