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When you’re pregnant, the nutrients and oxygen flowing through your veins doesn’t just have to nourish you—it’s also supplying sustenance to your baby. So, your cardiovascular system makes a number of changes to meet these increased demands, resulting in a change of pregnancy heart rate.
The amount of blood flowing through your veins in the first place increases by about half, says Margie Davenport, Ph.D., an exercise physiologist and director of the Program for Pregnancy and Postpartum Health at the University of Alberta. Each beat of your heart pushes more of that blood through your system, increasing what’s known as your stroke volume.
And—thanks to hormones and activation of your sympathetic nervous system—your heart rate also rises. The shift is gradual as your pregnancy proceeds, to the tune of 10 to 20 beats per minute from the first trimester through the third. By the time you give birth, your heart rate could be 20 to 25 percent higher than baseline. “It’s not uncommon for pregnant individuals to have a resting heart rate in the 80s, and even in the mid 90s,” Davenport says.
Taken together, these changes boost your cardiac output (the amount of blood your heart pumps each minute) by about 30 percent. And in a person who wasn’t pregnant, they’d also add up to high blood pressure. Think about it: You’re pushing more fluid, more frequently, through blood vessels that are much smaller than garden hoses.
But your body, wisely, adapts to accommodate them, making changes in everything from the flexibility of your blood vessel walls to the speed at which your kidneys can filter and flush waste products.
It’s true that some people do develop heart-related problems during pregnancy, in part because all the changes associated with the process act as a stress to the system. But the good news is that exercise, including running, with your doctor’s OK, is a great way to reduce your risk, protecting both you and your baby.
In fact, several years ago, Davenport and a group of colleagues reviewed all the available research on exercise and pregnancy to compile Canadian guidelines on the topic. They found pregnant people who were physically active were far less likely to develop many common complications, including gestational diabetes, high blood pressure, pre-eclampsia, depression, miscarriages, or an early delivery.
Watch power moms like 800-meter runner Alysia Montaño and Olympic Trials marathoners Rachel Hyland and Lauren Philbrook compete while pregnant, and it’s easy to forget that doctors once discouraged moms-to-be from moving at all, let alone intensely.
In fact, if you’ve heard that you shouldn’t let your heart rate rise to 140 beats per minute or higher while pregnant, you’re not alone: Davenport herself was given that advice when she was pregnant with her youngest daughter seven years ago. That number did appear in guidelines issued by the American College of Obstetricians and Gynecologists in 1985. But even then, it was based on limited data, and removed less than a decade later.
Still, the myth persists, to Davenport’s chagrin. In her mind, it prevents many women from taking advantage of the benefits of movement. So in her lab, she’s tested pregnant women’s heart rates on treadmill tests, calculating their maximum heart rates and heart rate reserve; with this data, she and her fellow researchers built a new table of heart rate zones for pregnancy.
|Age||Intensity||Heart Rate Range in Beats Per Minute|
“The caveat that we have in there is that we actually have relatively limited information about higher intensity zones,” she says. In part, that’s because the old guidelines prevented researchers from actually asking women to work out harder in studies.
As the number of athletes conceiving during the prime of their careers increases, scientists like Davenport are working hard to gather even more data. In the meantime, it’s always best to check in with your doctor about your exercise plans, and to clear any intense exercise beforehand—but know that, unless you have a complication like a ruptured membrane or uncontrolled high blood pressure, it’s generally safe to keep moving.
Gracey, who’s also a mom of two herself, agrees. Nausea and other symptoms made it challenging for her to run at all during her first pregnancy; during her second, she ran at least a mile every single day until she went to the hospital for delivery, even doing faster fartlek intervals through 14 weeks.
Her heart rate definitely went up more on hills, and took a while to come back down after fast intervals. So, she stuck to flatter routes and took more recovery time between harder efforts. While she did track her heart rate too, she primarily focused on keeping her intensity to no more than eight out of 10. “As long as my breathing could maintain a rhythm, then I was getting enough oxygen and so was baby,” she says—a piece of advice she passes along to her athletes.