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.