Whether you have done any yoga before or not, pregnancy is a time to get to into a different relationship with your body. The benefits of this are many; to prepare for changes in posture, useful positions for the later stages and techniques for birth, to ease and support the various niggles that can come up in the body throughout pregnancy, to reduce anxiety and difficulty sleeping, but also to meet other pregnant women and get some time away from your usual routine to take it all in.
Attending yoga in pregnancy is covered under 'antenatal appointments and relaxation and parent craft classes' therefore if you are in full time employment you are entitled to time off work to attend. You are welcome from anytime after 12 weeks to practice yoga, and to attend right up until birth if you wish to. No experience at all is required and nothing is expected of you.