During the first trimester the mother to be is not as restricted in range of motion so can do most postures, but what you may want to consider is not so much which postures to avoid, but how to do any pose. A general point of advice from teachers who have been pregnant and who are both prenatal and yin yoga teachers is to reduce hold lengths to 3 minutes max. Plus, however far the student could go before being pregnant, that should be the limit now. Even though hormones like relaxin may soften some of the fascia (pubic symphysis, SI joint ligaments, etc.) which may make more range of motion available, getting more flexible is no longer the intention of the practice. Use the practice to rest a bit, focus on sensations and/or the breath, and stimulate energy flow to both the mother to be and the baby.
Having said all that, there may be some postures which you may wish to avoid or at least modify: anything that puts a lot of pressure on the belly, like Sphinx pose, or really deep twists. These poses can be done (see the final bit of the video at the end of this description
of Sphinx for example) but modifications are suggested. See the list of Asanas
on this site for some examples of prenatal variations or pick up a copy of the second edition of The Complete Guide to Yin Yoga to see lots of prenatal alternatives. Also, this section of the Forum has lots of suggestions as well.
A last bit of advice: read this article
about the current understanding over whether women should lay on their backs for significant time when pregnant.
Good luck to your student!