In the past, the American College of Obstetricians and Gynecologists recommended that pregnant women keep their heart rate below 140 beats per minute, but more recent recommendations have eliminated such strict guidelines. Instead of focusing on the number on a heart rate monitor, know the signs you should look for in your own body. As a mom-to-be, you should never exercise so vigorously that you’re out of breath or can’t finish a sentence. Pushing yourself too hard can decrease blood flow to the uterus or raise your body temperature, which can lead to birth defects. Too much huffing and puffing can also lead to dehydration, which could put you at risk for premature birth. If you’re extremely thirsty, fatigued, have a headache, are dizzy or lightheaded, or have dark-colored pee, chances are you’re dehydrated. Remember that pregnant women typically need between 8 and 12 glasses of water per day, and even more if you’re exercising.