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.
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.
8 Pregnancy Myths You Need to Know, Pregnancy Exercise Guidelines - the Do’s and Don’ts