Hi Ella
In case you haven't seen my other posts littered around the forum, I teach Prenatal Yoga classes, so if you've got any questions then fire away.
My main advice is to find a teacher who has done some specialist training in this area and go to a specialist class (rather than just going into a general class and modifying) as many of the specific techniques that will help you through pregnancy, birth and beyond, will never be found in a general Yoga class. The other advice is to practice what you're shown in class as often as possible, Yoga really is a thing of repetition - the more you do it, the more benefit you will get.
I've had 10 babies so far this year (not personally you understand) and all the mums swear by the Yoga practices before, during and after birth. Some of them went without any pain relief at all and found the breathing practices especially indispensable. The classes themselves are good fun, light-hearted, and there's a good sense of community develops.
Hope you find a good teacher,
Scott