It is my belief that, the decision to practice yoga (or not) during the 1st trimester is an individual matter. With this said, I also believe that if you haven't practiced yoga before
, you should probably go with the advice of your instructor/studio, simply because you don't really have any other frame of reference. Certainly, it is not unusual for women to be told not to practice yoga during the 1st trimester. I think that this has historically always been case, in yoga tradition (not a yoga history expert by any means, so please correct me if I'm wrong!).
However - I've been practicing yoga for around 12 years now, and have continued to practice throughout my first trimester (I'm at 14 weeks today). For the last 18 months, I've been regularly attending an Ashtanga class, which is certainly quite physically challenging, but I have felt comfortable doing almost everything I was doing before I got pregnant, with the exception of some of the deeper back bends, deep twists, and upward facing dog. I also skip the occasional vinyasa, because I definitely tire more quickly than I used to :yawning.
Once I found out I was pregnant, I spoke at length to my yoga instructor about practicing during the 1st trimester. Her advice was quite simple: do what you feel comfortable doing (paying particular attention to the deep twists/backbends, as these can often be uncomfortable for pregnant women). She believes - as do I - that your body is the best gauge for what you should, and should not be doing. If something feels uncomfortable, don't do it. Her advice was based around the fact that because I have been practicing yoga for quite a long time, I was at a point where I could make the distinction between a challenging (but healthy) pose, and one that could be injurious.
Funnily enough, the founder of Ashtanga (Sri K. Pattabhi Jois) believed that women should not practice Ashtanga Yoga at all during the first trimester! However, he was not a woman and did not know the experience of being pregnant, and how capable pregnant women actually are, so I choose to ignore him in this instance
As I mentioned in my first paragraph, though - if you're entirely new to yoga, take it easy and follow the advice of your instructor. I don't think there is really a sensible alternative, and you certainly don't want to push yourself in a way that is damaging to you or your baby.