The ACOG and most doctors encourage 30 minutes of exercise per day during pregnancy. However, the mainstream message of running during pregnancy is still to take it easy. Almost everything I read suggested walk breaks, doing only easy runs, more rest days, and decreasing mileage.
The thought of taking it easy for nine months didn’t appeal to me. I know several women who happily run easy for the entire duration of their pregnancies. That is what felt best for them, which is fantastic. But is this approach best for everyone?
My Approach to Running during Pregnancy
Even though my body was changing with pregnancy, my mindset and desires did not. I enjoy working hard. Even if I didn’t compete or train at the same level, I wanted to enjoy some variety and harder runs. My identity as a runner was not a switch that I could just switch off. At first, I felt guilty. Was I being selfish? Would this make me a bad mother?
Then I asked my OB about exercise, explaining my pre-pregnancy exercise regimen. Her answer pleasantly surprised me: “Do what you did before. Listen to your body. If it feels good, you can do it. If it hurts, stop.” The current guidelines state that women who previously participated in vigorous exercise could continue during pregnancy.
I am treating this post as descriptive of my approach to running during pregnancy, not prescriptive. I’m by no means an expert on pregnancy with my data point of one. I know that my experience in one pregnancy is not representative of every woman’s experience, because every pregnancy varies.
But as a running coach who works with female runners, I try to understand the recent research. I’ve coached runners before during pregnancy, including one who finished her first half marathon in her first trimester. I strive to empower runners, especially female runners, with accurate knowledge. There are already too many myths out there about running during pregnancy.
There’s a lot to discuss on running during pregnancy, from relaxin and injury prevention to strength training to racing. I plan on writing about several of these topics. To start, I want to share my general approach to running during pregnancy.
My weekly workout logs show that I’ve continued doing hard workouts, mostly tempo intervals, during my pregnancy. I should note that the weather has been cool and dry. Recent workouts included 3 x 10 minutes at tempo effort with 2-min recovery jogs, 8 x 3 minutes at 10K effort (fast tempo), and 2 x 15 minutes at tempo effort with 2-min recovery jog. Here’s why I took this approach to running during pregnancy.
The Heartrate Myth
You’ve probably heard that your heart rate should not exceed 140 bpm during pregnancy. That’s completely false. The ACOG removed this from their guidelines in 1994, but many doctors still suggest it. According to a 2010 survey, up to 64% of OBs and midwives still believe and recommend this.
Let’s look at two data points: the practice of elite athletes and recent research. I find the practices of elite athletes incredibly informing. Elites have access to the most knowledgeable experts, including coaches and doctors.
In recent years, more female elite runners have trained and race during pregnancy. Shannon Rowbury and Gwen Jorgenson did workouts such fartleks, tempo runs, and two-a-days during their pregnancies. Alysia Montano raced the 800m in both pregnancies at 8 months and 5 months, respectively. Stephanie Bruce continued workouts and race throughout her second pregnancy. She shared a detailed account of her training in a 2015 Women’s Running article, describing how she ran workouts and race within her threshold heart rate zone, 179 bpm for her – much more than the 140 bpm myth. She ran a 1:23 half marathon at 19 weeks pregnant – 13 minutes slower than her PR, but also faster than her easy run pace.
On the research side, a 2012 study published in the British Journal of Sports Medicine examined fetal wellbeing during strenuous treadmill running. The participants were pregnant Olympic level athletes, all with 23-29 weeks of pregnancy. The researchers found that as long as the women stayed below 90% of the max heart rate, fetal heart rate was unaffected. When they did exercise above 90% of max heart rate, uterine blood flow briefly dropped for one-third of the participants. However, it returned to normal levels upon cessation of exercise. All participants delivered healthy babies.
A similar study in 2013 looked at the effect of moderate intensity and vigorous intensity exercise on three groups of women: non-exercisers, regularly active, and highly active, all at 28-32 weeks of pregnancy. The regularly active and highly active women ran on the treadmill at moderate intensity (40-59% of max heart rate) and vigorous intensity (60-85%). Five out of 15 (one-third) of the highly active women saw a brief dip in fetal heart rate. Once exercise stopped, fetal heart rate returned to normal. Almost all of the women delivered at term within a healthy birth rate range. All the babies were healthy.
Researchers cannot ethically find the exact upper limit of intensity of exercise during pregnancy. However, these two studies indicate that below 90% of max heart rate is a safe threshold for active pregnant women.
Most researchers and doctors suggest avoiding extremely strenuous exercise. For runners, this means workouts or race efforts that leave you gasping for air and completely exhausting yourself to finish. Overexertion is not advised, nor is exercising to a point that pain or discomfort occurs.
Hard Workouts during Pregnancy
Based on this data and my doctor’s advice, I concluded that tempo effort workouts were completely appropriate during pregnancy. Threshold pace (approximately hour-long race pace) is approximately 85% of max heart rate; most tempo runs are done at 82-90% of max heart rate (within normal weather conditions)- completely within the safe zone.
It’s important to note here the consideration of injury risk. Relaxin relaxes your ligaments starting in the first trimester. As a result, other tendons and muscles relax. If you are injury-prone, approach hard workouts with caution.
Some women even opt to keep doing track workouts during pregnancy and just scaling as needed. The tempo run isn’t a strict guideline either. It’s simply the effort that I decided was safe for me based on the evidence and my pregnancy.
What matters is doing the tempo run by effort or heart rate, not pace. Paces will be slower as pregnancy progresses, you gain weight, and your center of gravity shifts. A tempo run should feel comfortably hard, like a 6-7 out of 10. You should be able to speak in short phrases (such as “pace feels good”) without gasping for air. If you can do this, you are training at a safe effort.
I’ve always done my tempo runs by effort, not pace. Even as my pace slows, how I approach a workout has not significantly changed. I assess my effort regularly (every 2.5-5 minutes) with a quick talk test. I’m ready to stop if anything feels off. (I don’t use heart rate since I’ve never used it before. Instead, I trust myself to listen to my body’s signals.)
For most of the tempo runs, I did ~30 minutes total of tempo running. Since I was building back after injury, I structured the workouts as tempo intervals at first. Tempo intervals are a great modification for pregnancy. Recovery intervals help keep the heart rate in control.
In a sense, I did scale back my exercise intensity. Before pregnancy, I typically did two hard workouts per week. Throughout the first trimester, I only did one hard workout per week at a lower volume than before.
If I feel pain anywhere (especially with relaxin levels high) or cramping, I will stop. If my breathing becomes too labored or I struggle to finish, I need to slow down. How I feel after the workout will inform what I do for future workouts. But so far, I haven’t experienced any of these.
Granted, this is all in the first and early second trimester. I have not gained significant weight or starting showing yet. As pregnancy progress, workouts will change and eventually be removed from the schedule altogether.
Will running hard cause miscarriage?
No, not in healthy pregnancies. Miscarriages occur from chromosomal abnormalities or medical conditions.
One survey from the Danish National Birth Cohort examined retrospective self-reported data suggested that women who exercise in the first trimester were statistically more likely to miscarry. At a glance, that is concerning. However, the study itself even recognizes a lack of a causal link. Women who exercise during the first trimester are less likely to experience morning sickness; some women who do not experience morning sickness have a higher risk of miscarriage. The analysis did not consider significant factors such as pregnancy symptoms, pre-pregnancy health, or medical conditions. Correlation is not causation.
Think about exercise from this perspective. The guidelines recommend avoiding deli meat and soft cheeses because of listeria, which can cause a miscarriage. Of the pregnant women who contracted listeriosis, 20-30% of these cases resulted in a miscarriage. The incidence of listeriosis in pregnant women is 12 out of 100,000, which means likely only 3-4 of 100,000 pregnancies result in miscarriage due to listeriosis – that’s 0.03% of pregnancies (source). If running hard was remotely as dangerous as listeriosis, the guidelines would be more cautious.
If it doesn’t feel right to you or it worries you, err on the side of caution. But there’s no need to approach running during pregnancy with fear unless a medical indication indicates otherwise.
Again, this is what I’ve found that works for me. Some runners do interval workouts and race 5Ks throughout pregnancy and deliver perfectly healthy babies. Some runners choose to take pregnancy as a break and enjoy easy running or cross-training.
But remember that women are strong. We aren’t in some fragile or injured state during pregnancy. Runners are strong also, especially when they listen to their bodies. Our bodies are smart – they will let us know what we can and cannot do.
(Disclaimer: I am not a medical professional. Please consult your doctor about exercise during pregnancy and listen to your body.)
Did you run through your pregnancy?
How do you measure your hard workouts – effort, pace, or heart rate?