Here is why the 140 BPM figure is no longer used (these ACOG guidelines from 1985 were removed in 1996). It should not be assumed that all pregnant women working at 140 BPM (the same heart rate) are putting out the same effort level. Do not use heart rate monitors, as they are inaccurate during pregnancy. But do not dismiss your doctor’s recommendations, he/she could be recommending them for you and you need to listen to that. Especially if you are carrying more than one baby.
Heart Rate and Pregnancy – Your heart rate response changes throughout gestation. It increases in early pregnancy due to underfill, then falls gradually but continually throughout the latter trimesters. So, what is underfill? When you’re pregnant, you’re not just circulating blood through your own body, but through your baby’s body. Underfill is the condition when you don’t have the same volume of blood pumping through your heart as you would when not pregnant. As a result of less volume going through your circulatory system with each pump (beat) of your heart, you heart rate speeds up to compensate. If you exercised at a non-pregnant woman’s target heart rate throughout each stage of pregnancy you could easily under-work or overwork.
This is why the American College of Sports Medicine (ACSM) does not recommend using heart rate, but use “rate of perceived exertion” or RPE. The American Council on Exercise (ACE) says stay between 5-8 and ACSM says 3-7 on a 10-point scale. Below Moms Into Fitness has evaluated all and combined them—stay between a 5 &7.
Rate of Perceived Exertion
0 Feeling you get when sitting.
1 Activities like getting dressed.
2 Feeling you might get while doing laundry.
3 Taking a casual walk.
4 Walking briskly, but still maintaining conversation.
5 Feeling you get when rushing out the door.
6 Feeling you get when rushing up a flight of stairs.
7 Exercise while singing.
8 Slightly tiring exercise, but still speaking full sentences.
9 Feeling fatigue. Breathing hard.
10 All out exercise. Could not maintain for more than 30 seconds.