To ease your mind, you can wear a heart rate monitor, but the "talk test" is a safe gauge, too. (If you can't or couldn't maintain a conversation during the activity, that's your cue to back it off a bit). Of course, listening to your body outside of your pregnancy is necessary, too. If you're a runner, and all of a sudden you're getting knee, back and pelvis pain on what used to a simple route, adjust accordingly. Alternate a walk-run routine, or switch to indoor cycling, swimming or elliptical to give your joints a break. Remember that when you ignore what your body is asking for during pregnant exercise, you can create lasting back and joint issues to your own body, due to the change in your structural gait and biomechanics while pregnant. It's not about staying as good as you once were. Instead be the best you can be, safely, in this present moment.
If you don't have a regular workout routine, pregnancy is a great time to start! Get in the habit of walking for at least a half hour each night. Not only will help to build your energy levels, you'll combat the potential for gestational diabetes, and hypertension. Studies repeatedly show that people who are fitness-oriented make better eating decisions, too. If you hate working out, consider it a gift to your baby, and an opportunity to connect in a time of peace.
If you'd like the full details on studies regarding safe pregnancy exercise, read the full scientific study here.