During the first trimester, it's important to take extra precaution and not exert yourself too much, especially if you are a yoga novice.
Supta buddha kahasana
This pose will open the hips and help relax the lower back. It also will expand the chest area. For better relaxation, take a few deep breaths while you are in position.
The warrior II pose will help you stay active and healthy in the first months of pregnancy. This exercise also opens the hips and alleviates backaches.