How Fit Is Your Fetus? Exercise During Pregnancy and Fetal Heart Rate
Pregnancy made me tired – really tired. Pregnancy fatigue made me collapse into the couch at the end of the day (or heck, even at the beginning of the day), and it made the thought of getting up off that couch extremely painful. If I didn’t have to pee ALL the time, I might have been tempted to live on the couch full-time.
But then, there was a nagging voice in my head that said I should be exercising during my pregnancy. Yes, the couch was more inviting than the thought of taking my altered centered of gravity for a run in shorts that no longer fit. The trick for me was to fit in the exercise before the couch and I made eye contact. Going straight from work to the yoga studio, the gym, or a walking trail was the only way exercise would happen. And most of the time, the movement felt really good. I felt better about myself and my changing body, and I slept better at night.
Beyond these immediate benefits, women who exercise during pregnancy often have shorter labor and delivery times, fewer pregnancy complications, and faster postpartum recovery. Who isn’t motivated by the thought of those benefits? The CDC and ACOG recommend that healthy pregnant women get at least 150 minutes of moderate intensity aerobic exercise per week. That’s about 30 minutes per day, 5 days a week, of walking, jogging, swimming, or whatever floats your boat, within reason.
Exercise is good for a pregnant mom, but what about her fetus? How does the fetus feel about all this jostling about and heavy breathing? Many studies have shown that moderate exercise is safe for the fetus, and a new study indicates that when mom exercises, the fetus actually becomes more fit, too!
In a recent study, Dr. Linda May and colleagues at Kansas City University of Medicine and Biosciences and the University of Kansas have found that more intense exercise during pregnancy is associated with changes in fetal heart rate similar to that found in adults undergoing fitness training .
50 women at 36 weeks pregnant completed a physical activity questionnaire in which they reported what kinds of exercise they did and for how long. The questionnaire covered all of pregnancy and the three months before pregnancy, but the study focused on the data from the 3rd trimester. These women were not all stellar athletes. 36% were overweight or obese before pregnancy, and the median duration of exercise was just 20 minutes per day of moderate to vigorous activity during the 3rd trimester.
While the pregnant moms relaxed at the clinic, heart rate and heart rate variability were determined in their 36-week-old fetuses using magnetocardiography (MCG). MCG may sound scary, but it is basically a magnetic microphone that amplifies the electrical activity of the fetal heart through sensors on mom’s belly, giving a safe and non-invasive window to fetal heart rate patterns. Whether the fetuses were being active or quiet could also be determined by MCG, so the researchers could observe how their heart rates changed between the two states.
When the researchers lined up mom’s exercise with baby’s heart rate patterns, they found something pretty cool. The greater the intensity of mom’s exercise, the lower the fetuses’ heart rate when active and the greater the fetuses’ heart rate variability. Moms who exercised for longer during the 3rd trimester also had fetuses with greater heart rate variability.
What does it mean to have a lower heart rate but more heart rate variability? Well, to be honest, we don’t know what it means in a fetus. In children and adults, though, aerobic exercise training has the same effect – lower heart rate and but greater variability . These measures are not only associated with better fitness but also with protection from cardiovascular disease [3, 4].
The study showed a correlation between maternal exercise and fetal heart rate, but we don’t know for sure if exercise caused the differences in fetal heart rate. For example, fetal heart rate and a mom’s tendency to exercise could also be related to mom’s genetics or her nutrition. However, the researchers found that maternal age, resting heart rate, pre-pregnancy BMI, and pregnancy weight gain did not impact fetal heart rate. The fact that mom’s fitness level (indicated by resting heart rate) and BMI didn’t impact fetal heart rate makes it more likely that exercise during pregnancy is indeed an important factor.
Why care about fetuses’ fitness? That’s a great question. Future studies will need to determine if the fitness advantage persists after birth. Do women who exercise during pregnancy have kids who grow up to have better aerobic fitness and be better athletes? It is certainly possible. We know that other conditions such as maternal nutrition and stress during pregnancy can influence fetal development and alter disease risk later in life. These changes occur via epigenetic mechanisms – that is, they change the way genes are transcribed without changing the underlying DNA code. Exercise during pregnancy could have a similar epigenetic influence on the fetuses’ level of fitness in the womb and later in life – and perhaps lower the risk of cardiovascular disease.
This is a new area of research, and more studies will reveal if exercise during pregnancy has a lasting impact on our kids. In the meantime, the thought that your fetus is getting fitter with you might be that added push you need to get off the couch and get your pregnancy exercise on. Don’t worry, the couch will still be there, and it will be all the more inviting when you’re done.
Did (do) you exercise during your pregnancy? Is is motivating to think that exercising during pregnancy might give your child better fitness, too?
1. May, L.E., R.R. Suminski, M.D. Langaker, H.W. Yeh, and K.M. Gustafson. Regular Maternal Exercise Dose and Fetal Heart Outcome. Med Sci Sports Exerc. 2012.
2. Mandigout, S., A. Melin, L. Fauchier, L.D. N’Guyen, D. Courteix, and P. Obert. Physical training increases heart rate variability in healthy prepubertal children. Eur J Clin Invest. 32(7): p. 479-87. 2002.
3. Thompson, P.D. Exercise and physical activity in the prevention and treatment of atherosclerotic cardiovascular disease. Arterioscler Thromb Vasc Biol. 23(8): p. 1319-21. 2003.
4. Billman, G.E. Aerobic exercise conditioning: a nonpharmacological antiarrhythmic intervention. J Appl Physiol. 92(2): p. 446-54. 2002.