It is important to keep your heart rate lower, but you should be able to maintain the same activities (Lifting,running, etc.) into the second trimester
. The key is to listen to your body and stop if you feel any pain. I ran until about 30 weeks and did sit ups through the first trimester and feel like that has been a large part of my complication free pregnancy (so far at least). I am 38Wks+2.