What I have heard is not to overdo it----- exercise will increase your strentgth, flexibility, endurance, etc for pregnancy and can help you to have an easier labor. What I have heard is to not do anything new--- and maybe take your workout down a notch or two. Your heartrate should stay around 130 consistently- I usually take my heartrate up to 150 and work out for 50 minutes 5x- 7x/ week. After talking to my doctor, my goal during pregnancy is to work out four days a week for 40 minutes (including a longer warm up and cool down) and keep the heart around 130. I have had three miscarriages-- but the doctor does not think I should stop working out altogether. If your heart rate goes to high, your body temp raises too much and also the body goes into "self preservation" mode. On the other hand, working out can help to keep estrogen levels from getting too high.
I hope this helps-- and good luck!