You need to be careful when doing yoga in the second trimester. Hold on to poses till the time you are comfortable. Do not pressurize yourself. Remember you are expanding and therefore it will affect the sense of balance. Use pillow to raise yourself when lying down and never exhaust yourself.
In third trimester your belly is bigger than before and therefore do all those heel poses with a support. A chair or a wall can help you keep your balance. Use blocks or straps to move and change different poses with stability. Do not stand still for a long time as it slows down the blood flow back to the heart in pregnant women.
Do not lie on your back after first trimester for more than 30 sec. It creates pressure and reduce blood flow to the uterus. It can also make you feel dizzy and breathless.
Skip yoga position that requires extreme stretching. Do poses which your body permits. Do not influence yourself seeing celebrity doing extreme yoga poses in their pregnancy. Not every pregnancy is same.
Lastly avoid exercise or yoga classes in humid rooms. Overheating of body is dangerous for you and your child. So be safe and practice yoga and exercise till the time you are comfortable.