From: http://prenatalyogacenter.com/blog/yoga-in-the-first-trimester/

Prone poses. Certain belly down poses are fine, like bhujangasana (cobra pose), as long as the practitioner is grounding the pubic bone and elongating through the lower back and actively using her legs, the lower belly is not getting direct pressure. However poses like danurasana (bow pose) and salambhasana (locust pose) both put direct pressure on the uterus and should be avoided.

Deep twists. This family of poses is wonderfully therapeutic for the nonpregnant body since it provides an excellent means of compressing the internal organs which stimulates and cleanses the organs. Even though the uterus is still fairly small and situated in the pelvis, I recommend avoiding deep belly twists. Instead focus on twists of the upper back, keeping the “baby” pointing forward and twisting above the bra strap line.

Deep backbends. These poses should also be approached with caution since they too, compress the uterus and can over stretch the abdominal muscles.

Jumps and jerky movements. These movements can disturb implantation.

Abdominal strengtheners. At the Prenatal Yoga Center, we do not teach abdominal work to first trimester students. My reason being, that during the first trimester there is the high incidence of miscarriage. I have never come across data that states abdominal work will cause miscarriage, but I know that many women are protective of their belly region and concerned about any hardening activity of this area. I would not want anyone, should they suffer the loss of their pregnancy, to think that the ab work they did somehow caused the miscarriage.

We do focus quite a bit on transverse abdominal work in the second and third trimester. We believe abdominal work at this time in pregnancy aids and prepares the mother for the second stage of labor (pushing) and supports the back as the uterus continues to grow.

Certain pranayamas. Avoid pranayamas that involve breath retention and deep forceful movements of the belly like kapalabhati or bhastrika (bellows breath) Alternate nostril breathing is fine as long as you DO NOT add retention. Ujjiyi breath should not be practiced if the mucus membranes are swollen causing stuffiness. Most laboring women breathe in through their nose and out through their mouth. Since the one of the focuses of prenatal yoga is to help prepare for labor, I encourage the students to practice mouth breathing in their asana practice since it will be familiar to them for their labor and birth.