It is always a good idea to ask your doctor before engaging in any physical activity during pregnancy. Once you have approval for yoga, you might try looking for a prenatal yoga class so that your activity is better tailored for your needs and expanding midsection.
Once you enter your third trimester, there will be poses that are not good for you or are too difficult to accomplish. This include the camel poses, upward bow, any that require balancing on only one leg, back bends, or lying down on your stomach or back. Focus on standing poses or any poses that you can do on your knees or while lying on your side.