If you are in the habit of excercising then it shouldnt be an issue. Doctors actually encourage you to be active throughout your pregnancy. It makes your delivery easy, lets you get back in shape soon after birth, etc. Yoga and treadmill should be just fine, also make sure you are eating healthy and not starving yourself to lose weight.
There is more information on the net regarding this. I mostly get all my questions answered through babycentre.com