There's not a great deal of evidence based material when it comes to yoga so it's exciting when something new shows up. In an article in Physiotherapy Frontline magazine (5.9.18) it identifies an article posted by the Journal of Obstetrics & Gynecology: [https://obgyn.onlinelibrary.wiley.com./doi/10.1111/jog.13729] where it says, "Maternity yoga may help to crub stress levels and improve night-time sleep" shown in a comparative study of 91 women in their third term of pregnancy. The findings show, "It seems to activate the parasympathetic nervous system and decrease a-amylase levels." So get out there and join a yoga class!
It's best to join a class that is taught by a teacher who is qualified to teach pregnancy yoga, or has at least got training on encorporating pregnant ladies in their yoga class. British Wheel of Yoga teachers cover this last point in their basic training. The British Wheel of Yoga also has a post grad module on teaching yoga for pregnancy, these teachers run specific classes just for pregnant women.