If you have been doing yoga regularly for at least one year prior to your pregnancy, then you may continue the 26 posture series until your third month, or up until the time you are no longer able to lie on your abdomen. Get a hold of Rajashree Choudhury's pregnancy yoga video and start learning the pregnancy series during your first trimester, so that should you feel the need to adapt for your pregnancy before the first trimester is over, you can do so. Please read all the information on this site, and discuss it with your doctor or midwife.
Once you have entered your 2nd trimester, it is recommended to start practicing the pregnancy series in class. This will get you used to your body in poses as it starts making its changes.
You can practice the pregnancy series in a warm room with a doctor's approval. Ways to modify intensity level: stand in the cooler spot of the room (next to front door), break if feeling over-heated and sip your water.
If you have not practiced Bikram's series for at least one year prior to pregnancy, or you have a high risk pregnancy (history of complications) then you should stop practicing the series for the first trimester. You can practice the pregnancy series in a warm, but not hot room (between 85-95°F) with a doctor's approval.