from the perspective of my training as a yoga teacher...
Postures to consider avoiding:
inversions of all kinds - including downward dog(especially if you are new to yoga), though there are some inversions recommended later in pregnancy for certain situations. Legs up the wall is great for lowering blood pressure as well and is an alternative to inversions.
twists - which are cleansing in nature and can create extra toxicities in your body which is not good when carrying a baby
backbending (excessively) - this can cause separation of the rectus abdominus muscle which creates a situation following the birth that can lead to an increased hernia risk.
supine postures - laying on the back can increase the risk of compressing the inferior vena cava once the baby is of significant weight. You'll know if you can lay on your back or not...listen inwards always!
Generally enjoy postures that open the hips, strengthen the legs and encourage you to feel prepared to birth a baby. I really enjoy the Chandra Namaskar (moon salutation) during both my moontime and pregnancy. A google search is sure to show you this sequence...if not I'll look online for a good resource.
Also, I have heard of women practicing hot yoga while pregnant...this can be dangerous in the sense that it elevates the body temperature significantly. I would avoid this at all costs. But again...despite any recommendations, yoga is a practice of listening inwardly and understanding and knowing the body and mind connection...trust in what you hear.
http://yogalina.wordpress.com/2009/1...on-salutation/