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 [1].

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 [2]. 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?

REFERENCES

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.

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29 thoughts on “How Fit Is Your Fetus? Exercise During Pregnancy and Fetal Heart Rate

  1. I have always incorporating exercise into my daily life – I love the health benefits – both physical and emotional. Throughout my first pregnancy, I did moderate exercise, some walking, a bit of running, and a little yoga. However, with this pregnancy (which, hello…I’m so ready to end!) I ran a marathon at the very end of my second trimester and participated in a intense ballet/Pilates class a couple times a week. I’m really interested to see (and was curious before reading this article) if my newborn will be a more endurance-type athlete because of all the training I did while pregnant! And, if this will affect my labor and her birth weight.

    • A marathon?! Wow, you are my hero! I think I stopped running around 24 weeks, and by then, I wasn’t running more than a few miles at a time. I had planned to run longer, but my knees were starting to feel funny, and the Arizona (where we were living then) heat was getting to me. I switched to swimming, which I’ve never really liked, but it was great for pregnancy, particularly in that heat! Anyway, you amaze me:) Best of luck with your birth!

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  3. Interesting post! And I guess for most of us (at least those who are pregnant or with young kids), it is probably too soon to tell the outcome of any personal experiments. This, my third pregnancy, has been my swimming pregnancy, due to some uncomfortable varicose veins. My second pregnancy was my walking pregnancy. And my first was probably my least active pregnancy as I was still working at a desk job full time, but I did do some walking and yoga. Of course, to confound things, my first is a girl and the subsequent two are boys, so there may be gender differences involved too.

    In all honesty, my primary motivation of exercise during pregnancy was relaxation and a bit of alone time (which in the case of kids under 5 years old, go pretty much hand in hand!), rather than calorie burning or to increase fitness (of either me or the baby!). I suspect I burned more calories/ elevated my heart rate more by running around after two little ones than notching up the laps in the pool! I certainly think being active during pregnancy is a good thing, but then again, it is during any stage of life, and do pregnant mums really need any more pressure to adopt the perfect lifestyle during the pregnancy for the benefit of their offspring? For those who enjoy exercise, they will likely go on enjoying it during pregnancy. If it’s a chore, well, it will probably become even more of a chore. Nothing like being told you really *should* do something to make you not want to at all! It may also depend on access to resources such as babysitting, gyms or pools. Or even what time of year it is!

    Anyway, I digress. I did indeed exercise (varying amounts and types) during all my pregnancies. While it’s nice to know it may have some benefits for the kids, I’m not sure I find the knowledge extra motivating. Interesting to see how my kids turn out though… ask me in about 5-10 years time!

    • We never really know what goes into the mix for each kid. It is a roll of the dice genetics-wise, and we are really only beginning to understand how the conditions during pregnancy affect our kiddos. I don’t mean that in a scary way, but in a neato way:) I agree that pregnant moms shouldn’t feel pressure over this sort of thing. For me, though, it did help motivate me to know that exercise might help me prepare for labor. I just found it cool to think that my baby could have some kind of cardiovascular benefit as well. And most of us, no matter why we choose to exercise, feel better after we do it! Whatever the motivation, it is a good thing – as is relaxation and alone time! I’m glad you’ve been able to fit it in. I can imagine how hard it is to carve out the time when you have two little ones to juggle (and I agree that you probably get quite a workout trying to keep up with them!)

      • Incidentally, it’s now three little ones (as of 2 days ago!). The labour lasted 45 minutes and only about 10 or 12 contractions! Thankfully, the final 3 contractions were actually at the birth suite of the hospital. Thankfully too, it was the middle of the night and there was no traffic.

        Did exercise during pregnancy help speed the labour? I suspect it was more genetics (my mom had quick deliveries too), and the fact it was my third baby (body seemed to know what to do). I’m certainly hoping to get back into gentle exercise as soon as I feel up to it. And congrats to those who manage marathons while pregnant – that’s impressive! I look forward to hearing about how their deliveries go!

        • Sarah, congratulations! Glad to hear that your little one is finally here and that your labor went so smoothly. I’m sure you’re right that there are many factors that contributed to the speed of your labor, but I bet that the exercise didn’t hurt and will probably help you recover faster, too. Congratulations again, and thanks for the update!

  4. Great post! I am 32 weeks pregnant currently and I have been averaging about 4 miles of running a day. This post answered many of my questions and makes me feel much better about my choice to continue running through this pregnancy. Thanks!!!

  5. Thank you for this post– it is so timely!! I’m at 23 weeks and, though I’ve changed my routine a bit (from heavy extended cycling and weights to brisk walks and yoga), I still try to get some exercise in every day. I have wondered if I’m overdoing it, or if there’s a time when I should stop, so it’s good to be reassured that exercise is something I should keep up with throughout the duration of my pregnancy. This is my first child and while the possible benefits of a shorter labor and recovery, etc. are very appealing (!), I’m glad that at the very least, I’m not hurting either of us.

    • I agree – the benefits of shorter labor and faster postpartum recovery were what I found most motivating! Everything I’ve read says that exercise is safe during pregnancy (so long as you aren’t scuba-diving), and I think your body will tell you when you need to back off a bit. I loved yoga during my pregnancy!

  6. I’m currently 5 months pregnant. Ran a marathon in Hawaii in December and felt great. I still run about 3x a week and do a bootcamp for functional fitness 3x a week. This is my 2nd pregnancy so I will be very excited to see what difference this makes for labor. I’ve been keeping a blog for my marathon training since Jan. 2011 and now I have changed the focus to my running through pregnancy. I’ve found some great studies and books as well that I have integrated into my posts. I love finding info that promotes women to stay active w/out fear throughout their pregnancies. This was a great post!

  7. Pingback: Do Chocolate Lovers Have Sweeter Babies? A Review and a Giveaway! | Science of Mom

  8. Pingback: Time To Get Fit Again « Nick's Blog

  9. I teach prenatal and postnatal yoga and so have always advocated keeping active during pregnancy, although I have always believed that gentle to moderate exercise would be most beneficial. I did at least an hour of yoga every day when I was pregnant, as well as walking and swimming, but did adapt my practice to suit the changing needs of my body. I didn’t at the time really think about possible fitness benefits to my child, but certainly believed that the feel-good hormones released by appropriate exercise, as well as by relaxation and meditation, would benefit my child as well as myself.

    • I LOVED yoga during my pregnancy, and in the months post partum, too. (Currently struggling to fit it into my life, and I miss those quiet practices when I was pregnant.) I definitely think that yoga helped with my labor, and it was also just a time to quiet my mind and feel close to my baby, something I’m sure benefited us both. Whatever exercise you choose during pregnancy, it is all about listening to your body and your baby and doing what feels good – which can change every day!

  10. I’m impressed with the marathoning Sara! I signed up to run a marathon a week before baby was concieved, and I don’t think I’m going to make it. The marathon is in July, baby is due in September. I’m still running, but taking it slower and I haven’t done anything over ten miles. Thanks for this post and the extra motivation to stay active even when exhausted. I’ll have to try prenatal yoga.

  11. I trained for a half marathon during my first trimester – completing my race at 12 weeks. Then I continued to do kickboxing and yoga (with little modification) until the day my son was born. I actually did a 1 hour kickboxing class the day my water broke. I also went on regular hikes. The days I missed my workouts, I felt terrible. Exercising made a huge impact on how I felt throughout my pregnancy. I only gained 25 lbs during pregnancy and by 6 months post-partum I was down 35 lbs with little to no exercise.

  12. I started a workout video at the start of my 2nd trimester (when my OB said I could “slow down” a bit with the weight gain!) and it ended up becoming a favorite part of my daily routine. I gained about 30 pounds and lost almost all my weight soon after the birth of my 8 pound 6 ounce girl. However, my breast size hardly changed at all in the last two trimesters, and I struggled with low milk supply and we are now drinking almost exclusively formula at 3 months :( I have wondered whether my exercise, relatively small gain (I think quite a bit of it was muscle) and quick weight loss contributed to my low supply. Anyone else have that experience?

  13. Personally, I was constantly sick during the first 16-18 weeks, to the point that I had to be taken to the hospital to get rehydrated. Trying to exercise was out of the question as I fainted in lack of, well everything the body needs, and there were two of us in need. Trying to swim later gave severe pelvic girdle pain and made me waddle like a duck, not very dignified. Bicycling worked pretty well apart from the fact that it’s considered dangerous for the fetus (because of the shift in balance that may cause you to fall more easily). So I had to keep to the waddling in the end.

    Anyway, from what I’ve heard – like Babycenters recommendations, ( http://www.babycenter.com/404_what-activities-should-i-avoid-during-pregnancy_7229.bc ) recommendations are moderate exercise. Also the gynecologist of a friend critizised her for jazz dancing for too long during pregnancy – supposedly it made the pelvic floor muscles too tense when in fact they need to loosen up a bit before birth. At least this gynecologist claimed there was a risk for trouble giving birth related to too tight pelvic floor muscles. And if in fact they are loosened the way they need to be, then you create other problems when burdening them with certain exercises.

    Generally I get a bit worried by mothers-to-be working themselves hard during pregnancy. I know a couple of sad prenatal birth-stories and other “underweight-newborn-baby” stories that could be related to this, but like you say, there is limited research on fetuses, for good reasons. But I don’t think our vanity should dictate our behavior throughout pregnancy. There is a baby to care and provide for, and to share a body with, and there are risks involved when mothers overdo things. Like the risk of overheating or dehydration or decreased blood flow to the uterus.

    I doubt they would recommend marathon running, as I suppose it is exhausting even if you’re a superwoman to begin with. If there is stress involved to the mother’s body, it will be stressful for the fetus’s body too. How about lactic acids affect on fetuses, or cortisole, or ammonia? And how does lack of energy and oxygen during excessive exercise affect the fetus? Just asking.

  14. I did crossfit during my entire pregnancy and would go for 2-3 mile runs during my first 2 trimesters, and shorter 1/2-1 mile runs during my third trimester. Regarding crossfit, I only continued this after checking specifically with my OBGYN regarding the safety of the movements and intensity of the exercise. I was surprised to hear her say that the biggest indicator of whether the exercise was too strenuous or not was going to be my own response. I had read previously that there were maximal heart rates not to exceed, but she informed me that current research supports mothers continuing to exercise at the same intensity they were exercising prior to becoming pregnant and modifying as exercise if no longer “comfortable”. Obviously for people who are used to pushing their limits during exercise, it can be difficult to distinguish uncomfortable from challenging. I wish that that guidelines were a bit more unambiguous when it comes to exercising while pregnant only because I think this would have helped me not worry as much. But I digress…

    My baby always had a lower fetal heart rate and often times us techs and my OB would interpret this a him sleeping only to observe movements and kicks and find that he was fully awake. He was born healthy and I stayed healthy with no complications during my pregnancy. I think I was personally more emotionally healthy and able to recuperate faster after birth because I had been able to exercise throughout my entire pregnancy.

  15. I am in my 25th week of my second pregnancy. I worked out intensely during my first pregnancy and continue to do so now. I do bootcamp/cross fit style classes 3-4 times per week. I often get stares but mostly receive positive comments about being amazing to still be able to keep up. I don’t feel like pregnancy should be treated like a medical condition or as if we are too fragile to exercise as we did before. I know what my body can handle and I know my limits. I would never put my baby in jeopardy. Working out makes me feel good and as long as it does, I will keep it up. Here’s to being a fit mama!!

  16. Pingback: 7 Amazing Reasons to Exercise Pregnant | Mom Closet

  17. This is a great post! As I struggle to exercise through my third trimester, lesser-known details about the benefits of working out really help to keep me motivated.

    As the mother to a toddler, I sometimes feel guilty taking time out to exercise. It helps to remind myself that it’s not just about me….it’s about the health of my growing baby and, in the long-run, about giving my kids the benefits of a healthy, well-balanced mama.

    I linked to this post in my 7 Amazing Reasons to Exercise Pregnant post on momcloset.com.

  18. My experiment in exercise during pregnancy is actually a little older than some (or all) of these. My daughter was born in July of 1994. I ran 3-5 miles whenever it fit into my life in the first 7 months or so. In the last month or more I ran most days, and usually about 5 miles (I was trying to make myself go into labor in the end, but it didn’t work out well.) My daughter got out of the baby jogger in kindergarten and ran along with my husband and me for 3 miles — but just that once. Normally she jumped out and ran for short periods. In 6th grade she joined cross country and won some races. By the end of high school she had 17 or more state titles and ten Maine high school girls All Time records, like a 10:13 for a 3200 in outdoor track. She’s now running Division I track and cross country at Iowa State University on a full scholarship. I have always thought that running before she was born was probably helpful, so that’s what I was searching for today when I found this article. Interesting! She also has a very low resting heart rate. She went to the hospital once and the heart monitor alarm kept going off because her resting heart rate was in the 30′s. They had to reset the machine. I’d definitely recommend exercise in pregnancy, although I think it’s smart to do what feels right for your own body. But I didn’t go into labor until I skipped a day of running — I tell her now that she only decided to be born because she was unhappy that she had to go without her run that day. It still bothers her to go a day without running!

    http://me.milesplit.com/articles/90539-hs-bloggers-bethanie-brown-waterville-hs-3

  19. During my pregnancy I worked out a minimum of 5 days a week running around 10-15 miles and doing low weight despite my old school doctor telling me to stop. It made me feel much better about my pregnancy, minimized my morning sickness, and overall I had a very healthy pregnancy and gave birth to my first nice big baby one day before my due date. I had a long but easy delivery without pain medication.

    I went with a midwife for the majority of my prenatal care and after we had a surprisingly big baby (he was 8 lbs 15 oz–but took a gigantic dump as soon as he was delivered so midwife said if he hadn’t done that he probably would have been closer to 9 lol). He was bigger than all the babies in my and my husbands’ family, and my midwife said there was study indicating intense aerobic exercise in the FIRST trimester results in a healthier placenta and is correlated (or causes) a larger baby. She said no such correlation was found in exercise in the second or third trimester, so suggested for any future pregnancy that I do nothing or much less in the first trimester. I asked her for a reference for this study but she never got back to me. I have since been asking pretty much every mother I meet and have seen no such correlation (some were QUITE active the entire time and had smaller babies). Has anyone heard of this study? Is this PURELY correlational or even conjecture?

    • James Clapp’s book “Exercising Through Your Pregnancy” contains the same information you just mentioned, and citations.

      I read his book as soon as I found out I was pregnant because I enjoy exercising. Aerobic exercise is well attested to in the literature, but weight lifting data is very limited. I hope that changes in the future as more women are doing weight lifting (popularized through cross fit). In the meantime, I’ve found myself a personal trainer who has previously trained pregnant women to handle my weight lifting programming since I no longer feel confident in doing it myself.

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