Researchers at the University of Alberta have confirmed that high-intensity resistance exercise during pregnancy is safe and well tolerated by both the mother and the fetus — evidence that could lead to updated guidelines for pregnant women.
"There have been theoretical concerns that if you lift too heavy, your blood pressure will get too high and cause a reduction in the amount of blood flow going to the baby," says Margie Davenport, director of the Program for Pregnancy and Postpartum Health and senior author of the recent study published in the British Journal of Sports Medicine. "However, it's important to actually test theoretical concerns, not just guess, to determine if that's a real concern or not."
Although light to moderate resistance training is encouraged for pregnant women, current guidelines typically discourage higher-intensity training because there isn't enough evidence that it's safe.
This study is the first to measure direct maternal and fetal responses to this type and intensity of resistance training. It follows a previous study led by Davenport that included an online survey about the amount of weight participants lifted and their pregnancy outcomes.
"The difference with this study is that we weren't relying on people providing the information. We were actually able to measure whether or not the fetus had a change in heart rate or a change in blood flow during the resistance training," says Davenport.
The study involved 20 participants — 10 pregnant women and 10 women who weren't pregnant — who were asked to perform three exercises: barbell back squat, bench press and deadlift. The exercises were completed at increasing intensities. The bench press was designed to test the effect of training while lying on one's back — something that is typically discouraged for pregnant women. The other two exercises involve large masses of muscle and thus "create a fairly strong stimulus," says Davenport.
Participants were also asked to do the Valsalva manoeuvre, a breathing technique used for safe weightlifting. "It's a performance manoeuvre that helps you lift heavier weights, but it's also protective of the cardiovascular system," explains Davenport.
"Previously it's been discouraged to lift heavy weights, to use the Valsalva manoeuvre, but there's not a lot of empirical evidence around it," says Amy Moolyk, master's student in the Faculty of Kinesiology, Sport, and Recreation and first author on the study. "Ours is the first study to look at these movements and using this breathing technique for these high-intensity efforts."
The researchers used ultrasound to monitor fetal heart rates and determine potential signs of distress. They also monitored umbilical blood flow markers, which "make sure the baby is getting an adequate supply of blood flow before, after and during the exercise to maintain fetal heart rate, oxygenation and nutrient supply," says Davenport.
Participants wore vests that allowed researchers to see their heart rate and oxygen saturation throughout the exercise. Their blood pressure was measured before and after the exercises, and researchers also monitored glucose and lactate through a finger prick test.
Davenport's research often includes elite athletes, and she noticed that an increasing number of them were becoming pregnant in the middle of their careers. Many chose to reduce their activity levels during pregnancy, but they typically did so because of a lack of information and evidence, not necessarily because it was the best choice for them.
"We are consistently being told they want to know how much is safe and what is unsafe," says Davenport.
It's a concern Moolyk knows from first-hand experience. As a former Olympic weightlifter, high-intensity resistance training has always been an important part of her life. During both her pregnancies, she was uncertain what level of activity would be safe. And as a clinical exercise physiologist at Connect Physiotherapy & Exercise, she often encounters pregnant clients with similar worries.
"We know that because we have such limited information on vigorous to nearly maximal exercise intensities, anyone who's even considering it is quite fearful and anxious if they do it," says Davenport. "Trying to reduce that fear and anxiety by providing evidence-based limits is really important."
"We want women to stay active during pregnancy, but our specific recommendations are lacking because we don't have the evidence," adds Moolyk. "This is going to contribute to providing better guidance and recommendations, and it can help women continue to engage in activities they enjoy."
The research received funding from an NSERC Discovery Grant and from the Alberta Women's Health Foundation through the Women and Children's Health Research Institute (WCHRI). Davenport is a member of WCHRI and holds the Christenson Professorship in Active Healthy Living.
