If you are reading this article, you probably arrived here by searching for one particular question: how should you approach running while pregnant? Are there changes you need to make to your running during pregnancy? This article seeks to provide evidence-based guidance to running during pregnancy. Always defer to your medical professional and listen to how your body feels above any of the suggestions listed below.
As a running coach, I have worked with dozens of women who ran throughout their pregnancies. Each woman has a different experience running while pregnant. Some stop early due to discomfort, others run up until delivery. Some stick to short distances; others have completed half marathons. I personally ran through the entirety of my only pregnancy, including the morning I went into labor. And while I’m done with pregnancy after one baby, I continue to work with pregnant runners regularly.
An Evidence-Based Approach to Running While Pregnant
The ACOG and most doctors encourage 30 minutes of exercise per day during pregnancy. The current guidelines state that women who previously participated in vigorous exercise could continue during pregnancy. If running is not new to you, most likely your doctor will clear you to run during pregnancy. Of course, you should always check first, as high-risk pregnancies and contraindications may be encouraged to refrain from high-intensity exercise.
Before diving into the evidence, it’s worth noting the limitations of the evidence. The nature of pregnancy limits how how studies can done. Many studies done on pregnant women rely on reported data, which can be prone to recall errors and other reliability issues. However, even evidence with its limitations is still valuable.
Let’s look at two data sets: the practice of elite athletes (also known as “results-proven practice” in the field of exercise science) and recent research.
In recent years, more female elite runners have trained and raced during pregnancy. The training stresses were nothing extreme for their bodies; they had adapted to faster running prior to pregnancy. Shannon Rowbury and Gwen Jorgenson did workouts such as fartleks, tempo runs, and two-a-days during their pregnancies. Stephanie Bruce continued workouts and racing 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 raced a half marathon while pregnant.
Should You Stay Below a Certain Heart Rate when Pregnant?
You may have 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 and coaches still suggest it. According to a 2010 survey, up to 64% of OBs and midwives still believe and recommend this. Of course, there is nothing wrong with recommending cautious approaches when making general recommendations. If you are not a trained runner, you do not want to start exercising at a high heart during pregnancy. For trained runners, though, the evidence indicates that the heart rate ceiling is outdated.
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.
Can You Do Hard Workouts during Pregnancy?
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.
The evidence suggests that moderately hard workouts (such as threshold workouts) are safe in healthy pregnancies of trained athletes. 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. (Here’s more on modifying hard workouts for pregnancy.)
Can You Run Long Distances While Pregnant?
There is no evidence indicating that long-distance running is harmful to pregnancy, unless medically contraindicated. A 2018 retrospective cohort study found no differences in gestational age or weight at delivery when comparing female runners to the general population. So long as you are increasing your energy intake, hydrating well, avoiding heat stress, and listening to your body, running for an hour or more is not demonstrated to be dangerous.
When it comes to running a marathon while pregnant, it is best to consult your OB and work closely with an experienced coach. There are many considerations for very prolonged exercise while pregnant: managing blood sugar levels, ensuring adequate hydration, monitoring intensity and heart rate, and minimizing heat stress.
Does Running While Pregnant Cause Miscarriage?
A common concern of any mom is whether or not an activity such as running could increase the risk of pregnancy loss. Running is not associated with an increased risk of miscarriage 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. 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). Logically, if running hard was remotely as dangerous as listeriosis, the guidelines would be more cautious.
Key Takeaways for Running while Pregnant
When you are pregnant, prioritize your hydration and nutrition, including adequate carbohydrate intake on runs longer than an hour. Your fitness prior to pregnancy will guide what you can do during pregnancy. Listen to your body and only run if it feels physically good during and after.
If it doesn’t feel right to you, err on the side of caution. If you experience any scary symptoms such as pain or bleeding, stop running until you talk to your obstetrician. But there’s no need to approach running during pregnancy with fear unless a medical indication indicates otherwise.
(Disclaimer: I am not a medical professional. Please consult your doctor about exercise during pregnancy and listen to your body.)