What Do I Need to Know about Exercise During Pregnancy?
I was completely over-the-top ecstatic to come across an article in this month's Sports Medicine relating to new recommendations for exercise during pregnancy (Zavorsky & Longo, 2011). My experience during my medical training thus far is that, with pregnant patients just as non-pregnant patients, there is not a whole lot of discussion about exercise intensity. It's either “it's safe to exercise, and you should” vs. “it's not safe to exercise; don't.” For some pregnant patients, this is adequate; for many, it's not. So when this latest contribution to the medical literature purported to provide recommendations for exercise intensity during pregnancy with specificity unlike any before it, I was beside myself. That's what I get for getting this excited over a journal article…
A bit of history: In 2002, the American College of Obstetricians and Gynecologists issued guidelines for exercise during pregnancy and the immediate postpartum period. While very helpful in terms of issuing absolute and relative contraindications for exercise, criticism arose in response to its lack of specificity about intensity and duration. In 2003, the Society for Obstetricians and Gynecologists of Canada took a stab at its own guidelines, and prescribed a thorough set of heart rate guidelines (as in, specific heart rates — imagine?) for women with uncomplicated pregnancies, theoretically based on modified calculations for maximum heart rate (Zavorsky & Longo, 2011). Sigh. So here we are in 2011, where the authors of this latest review of evidence are advocating for calculating heart rate reserve percentages (as in, another formula that includes maximum heart rate). You know how we at ICI feel about maximum heart rate formulas.
But rather than be disappointed about the lack of practicality that I feel these specific intensity guidelines provide for me both for the pregnant women I train or the pregnant women I see in clinic, I think there is still some general principles we can learn from my synopsis of all three sets of recommendations. Note: I will be speaking with reference to only uncomplicated pregnancies.
1. In the absence of medical contraindications, regular exercise during pregnancy is good.
There is evidence that it can prevent and help manage gestational diabetes, help reduce excessive weight gain during pregnancy (Zavorsky & Longo, 2011) – for women with normal pre-pregnancy weights, we aim for pregnancy weight gain of 25–35 lbs. All pregnant women should be evaluated by their physician to evaluate whether there are any absolute or relative contraindications to exercise (Artal & O'Toole, 2003; Davies et al, 2003). If you have riders in your indoor cycling class who are pregnant, you should ask them if they have been evaluated by their doctors and whether they have been advised of any restrictions. There are a lot of good books from respected experts 
2. ACOG has described warning signs that pregnant women should stop exercising. You should be aware of these warning signs in case you see them, and refer your clients to medical attention.
These include vaginal bleeding, shortness of breath before beginning exercise, dizziness, headache, chest pain, weakness, calf pain/swelling (I actually saw this during my 1st week as an indoor cycling instructor — I'd had no medical training at that time but decided it didn't look right, and told my client I thought she should leave class and call her doctor immediately. She did and, turns out, she had a deep venous thrombosis — or blood clot — in her calf. High levels of estrogen during pregnancy predispose people to blood clots.), amniotic fluid leakage, signs of preterm labor (Artal & O'Toole, 2003; Davies et al, 2003).
3. If I leave you with nothing else: Turn the AC/fans on.
Go dig up Gene Nacey's epic article about why we need ventilation to facilitate evaporative cooling during an indoor cycling class, and how dangerous it is to fail to do so. Take that, and magnify the risk for a pregnant woman. During pregnancy, core body temperature is elevated at baseline. During exercise, core body temperature rises further. If a pregnant women exercises in a neutral or cool temperature, as well as maintains adequate hydration and electrolyte balance, there should be no negative impact on the fetus. The best indicator of whether thermoregulation is going ok is the pregnant woman's subjective account (Artal & O'Toole, 2003). If she feels too hot, she is too hot and needs to change her environment; she should feel comfortable at all times, regardless of her (or her co-riders') desire to sweat buckets in your class.
4. Pregnant women need to accommodate for the normal physiological changes that occur to their musculoskeletal and cardiovascular (among other) systems. With knowledge about what is happening, modifications to exercise regimens should be based on common sense.
During pregnancy, connective tissue gets more lax (one might reason that looser ligaments are a pretty convenient adaptation, given what needs to happen during labor). With ligament instability in addition to weight gain causing increase force across the hips and knees, pregnant women may be more prone to musculoskeletal injury. Walking lunges may be particularly risky with lax pelvic ligaments, and should be avoided. Strength-training is best done with light weights/high repetitions (we know that women won't be accomplishing much in the way of muscle strength building — and that's ok. The idea is to keep building muscular endurance). Heavy weights or isometric exercises should be avoided, due to theoretical compromise in fetal blood flow. Women should also be advised not to lift anything while laying on their backs — this position puts pressure on an important vein called the inferior vena cava, which decreases cardiac output (doing the same exercise on an inclined bench is fine). Strength-training with resistance bands may be safer than free weights, given the possibility of inadvertently knocking into the abdomen (Zavorsky & Longo, 2011). Speaking of which: Contact sports should be avoided, as should activities that increase risk of falling (a stationary bike should theoretically be fine). With increased basal metabolic rate, pregnant women have increased nutritional requirements — she needs to replete even more fuel before, during, and after exercise than she did prior to pregnancy. Lastly, there are dramatic changes to the cardiovascular system: increase in blood volume, heart rate, stroke volume (how much blood is ejected from the heart with each heart muscle contraction) increase; the resistance within blood vessels decreases (leading to a drop in blood pressure). So even a well-trained athlete is going to experience a different response to the same intensity of exercise, and may need to scale back her efforts (Artal & O'Toole, 2003).
5. Stay aerobic.
For all the discussion of METs and % HRR and blah blah maximum heart rate formulas, it all comes down to this: all the research on safety in exercise, even “vigorous” exercise, during pregnancy is referring to intensity levels that we would still define as sub-lactate threshold, even if the researchers never call it that. I am not a doctor yet, but I will be recommending to my future patients that they “stay aerobic” during pregnancy. If you have a pregnant rider in your class where you are incorporating anaerobic intervals, I would strongly suggest encouraging her to modify her efforts. Use of a heart rate monitor during pregnancy is difficult — evidence is mixed for how HR responds to exercise during pregnancy (Zavorksy & Longo, 2011): some women have a blunted HR response (as in taking a beta blocker!); some do not. Most women have an increase in resting HR (Artal & O'Toole, 2003). Due to these irregularities, rate of perceived exertion (RPE) is the way to go.
6. Pregnancy is not a time for achieving peak fitness or any sort of hardcore training. The idea is to keep moving and keep comfortable.
30 minutes most, if not all, days at that “light” to “somewhat hard” RPE (Davies et al, 2003). If a woman is exercising longer or more intensely than that, the number of days per week can be decreased (Artal & O'Toole, 2003). If a woman had been previously inactive, she should begin with 15 minute sessions and gradually increase to 30 minutes (Davies et al, 2003). As explained earlier, attention should be given to adequate nutrition, hydration, and proper evaporative cooling (i.e., turn the AC on). In the absence of complications and medical restrictions, this should be continued throughout the pregnancy (Zavorsky & Longo, 2011).
You can go a long way with your pregnant clients just by asking if there is anything that is making them uncomfortable, and making common-sense recommendations for them to modify what they're doing. For example, early in my career I had a rider in an indoor cycling class who refused to acknowledge my gentle cueing to hold onto the handlebars during class. I was a substitute instructor and lacked confidence that I'd be able to accomplish anything, so decided to “let it go.” Then it occurred to me to ask her about it. “I can't help but notice that you're not holding onto the handlebars. How did you come to decide to do that?” She told me that she was pregnant, and that leading forward on to grasp the handlebars caused her a lot of pressure in her abdomen. I smiled and showed her that we could raise the handlebars so that she wouldn't have to lean over (remember: proper handlebar setup is only based on comfort!). She was thrilled! And just like that, we worked together so that she could enjoy her indoor cycling experience and derive all of its physical and psychological benefits — safely.
Both in pregnancy and non-pregnancy, the idea is that folks should listen to their bodies. We all have clients who struggle with this, and perhaps we struggle with it ourselves. In pregnancy, however, there is the “permissive” external motivation to decide when enough is enough — and for that to be perfectly okay. (Note: it's always okay.)
References
Artal R, O'Toole M. (2003) Guidelines of the American College of Obstetricians and Gynecologists for exercise during pregnancy the postpartum period. British Journal of Sports Med; 37:6-12
Davies GAL, Wolfe LA, Mottola MF, MacKinnon C. (2003). Exercise in Pregnancy and the Postpartum Period. Joint SOGC/CSEP Clinical Practice Guidelines, No. 129, June 2003
Zavorsky GS, Longo LD. (2011). Exercise Guidelines in Pregnancy: New perspectives. Sports Medicine, 41(5): 345-360
The author is in her final year of medical school at the University of Vermont College of Medicine in Burlington, Vermont. Author of the popular coaching motivation blog, Spintastic (http://spintastic.blogspot.com), her research interests include the psychological effects of heart rate training and use of heart rate monitors for anxiety treatment.
Have a medical research question for the ICI team? E-mail melissa.marotta@uvm.edu.