It is normal you should be rest assured. If you are already blessed with a child and this is the second time you will be conceiving or if you are expecting twins this time, then the blood flow to your heart will be even more, thus leading to a much faster heart rate. Ideally your heart rate should not exceed 140 beats per minute. When you are in your third trimester, as in the stage of last three months of your pregnancy, there is likelihood that there will be a tremendous increase in your heart rate approximately by about 10-20 beats per minute. It is during pregnancy that the need arises for inhaling more oxygen for fetal development to take place properly. Usually the resting heart rate of a woman is 70 beats per minute, but when pregnant, her pregnancy resting heart rate tends to increase up to 85 beats per minute. Fitness levels also act as major factors that help in deciding your target heart rate. Pregnant women should not opt for high intensity workouts; rather they should look out for moderate workout options like walking. They should workout for not more than 20-25 minutes at an average heart rate, which means about 50-60% of the maximum heart rate. So, exercise to keep healthy and fit, but take care, not to over attempt, because that can boost up your heart rate, making it hazardous for you as well as your baby.