I'll agree with psychounicorn above.
I felt pretty unstable, changed center of balance in the last trimester, and my joints/ pelvis started to feel looser.
Look for -
Pregnancy Workouts: How to Excercise During Pregnancy. Article by John Berardi, guidelines on heart rate safety, avoiding the valsalva maneuver, not laying flat on your back as in Bench Press, and safety guidelines for rep ranges and maximums.
New Mom Fitness by Nate Green is a nice article with pics of John Berardi's wife Amanda training when pregnant. She continued to lift, but did scale back a lot on the amount of weight she was moving.
My opinion, you can certainly exercise and lift when pregnant, but I'd err on the side of caution when it comes to anything that might put your baby at risk.
Yes, you can look on youtube and see pregnant Cross-fit women doing very strenuous workouts into the last trimester. I question the wisdom of that. If I had any doubts about something harming my pregnancy or causing premature labor, it's not worth the risk.
Wish you the best with your baby!