Yoga's wonderful to do once you're pregnant, but there are some poses to avoid.
Number one, Jumping to Chaturanga. Baby's locked in pretty well, but just in case you don't want to have a sharp fall to the floor. So avoiding that transition would be wonderful. Instead, simply step back to Plank and lower to Chaturanga from Plank.
Number two. Very obvious one, no closed Twist. That means nothing where your belly's going to be pressing against your thigh and you'll be using any kind of force or leverage to rotate the spine deeper. That kind of compression in your abdomen is not going to make your baby very happy.
Number three, you want to avoid deep back bends. Things like Wheel Pose will feel OK for only so long. As your belly gets bigger, you're increasing the risk of overstretching the abdominal wall and possibly tearing the abdominal wall. Simply back out of the back bends.
Number four, avoid prone positions. Don't be on your belly if you don't have to be and once you're uncomfortable, get off your belly. You don't need to be in your second and third trimester doing Locust Pose. There is plenty of other alternatives for strengthening your back.
Number five, you want to avoid overstretching in general. As your body produces more relaxin, a hormone that softens your ligaments, it becomes very easy to go beyond stretching your muscle and stretching to the joints and ligaments. The more you stretch the ligaments, the more unstable your joint becomes, the more off balance you're going to feel and the more risk for injury you're going to have. So keep the stretch in the belly of the muscle and if you're unsure, just back out.
And, finally, inversions. They're actually fairly safe to do up through the second trimester. Now starting in the third trimester, at some point the baby's going to start to turn down head first. Once baby has turned, no more inversions. You don't want baby to have the free will to turn again and get out of position. So once baby's head is turned and baby starts to drop into position, no more inversions.
Finally, any pose at all that ever feels uncomfortable, you don't have to do. If you're doing practice on yourself, skip it. If you're in class, skip it. Tell the teacher. Let them know you're uncomfortable with this pose and they'll generally offer you an alternative. Or if it ever feels uncomfortable, just take Child's Pose.