The biggest thing about doing regular yoga when pregnant is to ease up on twisting postures and don't do inversions due to the gravity pulling on your uterus. You shouldn't do inversion while having your cycle, either, but we don't have to worry about that anymore, do we?
Any yoga instructor worth anything will give you modifications for pregnancy. There are some things that I could probably still be doing, but why risk it? I've worked too hard for this baby to jeopardize anything!