I am writing this article for every Mom who has ever experienced a loss and for every Mom who has ever worried about harming their baby – so basically this article is for every Mom...ever. I will forewarn that some of the statistics I am presenting here here can be triggering and I encourage you to read this when you are ready.

I am writing with the best of intentions to help YOU make informed decisions in your pregnancy. I do not want you to fear exercise. I hope this article provides you with valuable information you can use to make the best decisions for YOU and YOUR baby. I would also like to make clear that I am NOT a healthcare provider, I am not a physician or anything even close. It is your responsibility to seek medical advice from a qualified medical professional. I have provided citations for the information presented here should you wish to read more in depth on any topic presented here.

Let me start by saying – Mama I see you, I hear you. You are strong, resilient, you are everything your baby needs you to be. Miscarriage is an awful experience; it doesn’t matter when it happens, there is no ‘easy’ pregnancy loss. There is no silver lining you need to look for and no “at least…” statements that can cheer you up. My only hope is that you have a strong support network you can lean on and that you feel comfortable sharing your story with your loved ones. I do not think miscarriage is discussed nearly enough. You are not alone. Miscarriage is tragic and it is also common. 

Let’s discuss.

Within the first trimester the terms miscarriage, spontaneous abortion, and early pregnancy loss are used interchangeably (1) and thus will be used synonymously throughout this article. Miscarriage, defined as loss of a pregnancy before the 20th week occurs in about 10-20% of pregnancies (2). Meaning as few as 1 in 10 and as many as 1 in 5 women will experience a miscarriage (3-6).

WHY DO MISCARRIAGES HAPPEN?

In most cases the cause of miscarriage is unknown. There are several known factors that contribute infertility and miscarriage. These include:

• advanced maternal age*

• obesity,

• smoking and alcohol,

• pre-existing medical conditions such as diabetes, polycystic ovarian syndrome, endometriosis and thyroid disorders are found in greater prevalence in infertile women and are linked to pregnancy loss

*Maternal age is the strongest known risk factor (7). A recent study from Norway looked at 421,201 pregnancies that occurred between 2009-2013, of which 12% ended in miscarriage. The risk of miscarriage was lowest among women 25-29 years old (9.8%), with absolute lowest risk at age 27 and the highest risk at age 45 (53.6%). 

WHY IS AGE SUCH A STRONG RISK FACTOR?

The older you are, the more likely you are to have a chromosomal anomaly that ends your pregnancy. In a 2012 retrospective cohort study of 353 pregnancies that ended in miscarriage, half of miscarriages in younger woman compared to nearly ¾ in those 35 years and older were due to a chromosomal anomaly (8). About 50-70% of non-recurrent miscarriages occur because of a chromosomal anomaly (9). 

Note: exercise has not yet been mentioned as a risk factor for miscarriage.

WHAT ABOUT RECURRENT MISCARRIAGES?

Sadly, recurrent miscarriage occurs in 3% of couples trying to conceive with 5% of couples experiencing two consecutive miscarriages (10). Women who have experienced a miscarriage are more than 1.5x as likely to have another miscarriage as those who have never been pregnant (7). Women who have experienced two consecutive miscarriages are more than twice as likely to experience another one.

This is where I am going to pause.

Mamas who have experienced 1, 2, 3 or more losses – my heart goes out to you. You are the strongest most resilient women in the world and your stories matter. You are NOT alone. Every loss matters. There is no silver lining to look for, it’s important to grieve your losses and understand that SO much of this is out of your control. Dads & partners – this includes you too. Although you may not have physically experienced the loss you are reeling from the emotions tied to it. You also deserve space for your feelings to be heard. 

IT IS NORMAL TO BE FEARFUL OF EXERCISE DURING PREGNANCY

In a 2016 literature review of 12 quantitative and 14 qualitative studies on perceived barriers to leisure-time physical activity (LTPA) during pregnancy mother-child safety concerns were 1 of the 5 most common themes reported (11). Consistently, women reported fear of harming the baby or themselves as a major barrier to engaging in LTPA – even those women who were physically active prior to pregnancy. Concerns around exercise safety actually predict the amount and/or intensity of exercise Moms feel comfortable engaging in (11) i.e. the more worried you are, the less likely you are to exercise. 

In fact, fewer than 15% of women will achieve the minimum physical activity recommendation for pregnancy (12). 

BUT IS IT SAFE TO EXERCISE DURING PREGNANCY?

The 2019 Physical Activity Guidelines for Physical Activity throughout Pregnancy (13) shifted our thinking from considering exercise as a recommend behaviour to improve quality of life to a specific prescription for physical activity to reduce pregnancy complications and optimize health for both Mom and baby. 

It is recommended that all pregnant women without contraindication be physically active throughout pregnancy. 

Specifically accumulating 150 min of moderate-to-vigorous physical activity per week over a minimum of 3 days (though being active every day is encouraged). There is strong evidence to support women engaging in a variety of aerobic and resistance training activities to achieve greater benefits.

There are two terms worth discussing: absolute and relative contraindications.

Absolute contraindications to exercise during pregnancy prohibit women from participating in strenuous activities. These include:

• Ruptured membranes.

• Premature labour.

• Unexplained persistent vaginal bleeding

• Placenta praevia after 28 weeks’ gestation

• Pre-eclampsia.

• Incompetent cervix.

• Intrauterine growth restriction

• High-order multiple pregnancy (eg, triplets)

• Uncontrolled type I diabetes

• Uncontrolled hypertension

• Uncontrolled thyroid disease

• Other serious cardiovascular, respiratory or systemic disorder

Women with the following relative contraindications are encouraged to discuss the risks and benefits to engaging in physical activity with their obstetric care provider. These include:

• Recurrent pregnancy loss

• Gestational hypertension

• A history of spontaneous preterm birth

• Mild/moderate cardiovascular or respiratory disease

• Symptomatic anaemia

• Malnutrition

• Eating disorder

• Twin pregnancy after the 28th week.

• Other significant medical conditions

Keep in mind it is still strongly recommended not to engage in physical activity that carries a significant fall or impact risk such as horseback riding, gymnastics, contact sports etc. You should also cease exercising immediately and seek medical attention if you experience: persistent and excessive shortness of breath that does not resolve on rest, severe chest pain, regular and painful uterine contractions, vaginal bleeding, persistent loss of fluid from the vagina indicating rupture of the membranes, and/or persistent dizziness or faintness that does not resolve on rest. 

That being said, there are incredible benefits to being physically active during pregnancy which include:

• 38% reduction in the odds of developing gestational diabetes mellitus

• 41% reduction in the odds of developing pre-eclampsia

• 39% reduction in the odds of developing gestational hypertension

• 67% reduction in the odds of developing prenatal depression and,

• 39% reduction in the odds of having a high birth weight baby (macrosomia)

• 24% reduction in the odds of having an instrumental delivery (14)

This is all without increasing the odds of preterm birth, low birth weight, miscarriage and perinatal mortality (15-19). 

WHAT ABOUT HIGH INTENSITY EXERCISE?

The 2019 Physical Activity in Pregnancy Guidelines recommend 150min of moderate-to-vigorous physical activity per week. Moderate being described as exercising between 40-59% of heart rate reserve (HRR) whereas vigorous activity is described as exercising between 60-80% of HRR.

Nerd sidebar about HRR – please skip ahead if this stuff puts you to sleep…

*Heart rate reserve is easily calculated: take your resting heart rate and subtract it from your maximum heart rate (maximum being roughly 220-your age). For example, I am 29: my maximum heart rate is ~191, my resting heart rate is around 52. 191-52 = 139 is my heart rate reserve. To prescribe exercise intensity you can use the Karvonen formula:

Exercise HR = % of HRR + HR at rest

Let’s say I want to exercise vigorously at 80% of my HRR. My calculation would go as follows:

Exercise HR = (0.8*139)+52 = 163bpm

I get that that sounds complicated. MOST women are better off using the talk test as a measure of perceived exertion. If you can maintain a conversation while you are exercising, you are exercising at a ‘moderate’ pace. Much above or below that would have you at a high or low intensity. 

SO WHAT DO WE KNOW ABOUT HIGH INTENSITY EXERCISE AND PREGNANCY?

Well for starters, we know that women who have high levels of physical activity prior to pregnancy are likely to continue to be the active during pregnancy (20). In a study of 4718 first time mothers, the number of women engaging in sedentary activity grew from 6% pre-pregnancy to 29% during pregnancy (20). There are a lot of reasons why women would be less active during pregnancy, main reasons being feeling tired or unwell, being too busy and especially in late pregnancy feeling too uncomfortable (11). Something interesting to consider – nausea is less common in pregnancies that end in miscarriage (21) but women who experience nausea are potentially less likely to want to engage in exercise. If we conclude that exercise causes miscarriage when really women who don’t experience nausea (which is more common in pregnancies that end in miscarriage) are more likely to be ones exercising, our conclusion is quite biased! Just a thought – now back to high intensity exercise during pregnancy.

Fetal well-being was assessed after a peak treadmill test in 45 healthy women (15 non-exercisers, 15 regularly active, 15 highly active) (22). All participants underwent a peak treadmill test to volitional fatigue. Fetal well-being was assessed before and after the treadmill test using umbilical artery Dopplers, fetal heart tracking/rate, and biophysical profile. While fetal-well being measures were reassuring for all 3 groups of women, one peculiar finding was noted. In 5 of the highly active women, transient fetal heart rate decelerations and elevated umbilical and uterine artery Doppler indices were reported post-exercise. Though all measures returned to normal it is curious that these findings were recorded in the highly active women group only. The clinical implications of such transient fetal heart rate decelerations are not known but it is possible that pregnant women – especially highly active women – can push themselves beyond a threshold where the well-being of their growing fetus may be compromised (22). To be frank – we really do not know whether exercising above 80% of max heart rate is good, bad or neutral for the growing fetus.

We do know that exercising between 80-90% of max heart rate into the third trimester is seemingly safe for most healthy pregnancies. A 2019 systematic review and meta-analysis of 13 studies reported no significant difference in risk of small for gestational age infants, risk of low birth weight or maternal weight gain for mothers who engaged in vigorous physical activity compared to those who did not (23). In fact women who exercised vigorously in pregnancy had a reduced risk of premature birth (though their pregnancies were slightly longer than women who did not exercise vigorously) (23). Note this review only included studies where moms exercised to a maximum of 90% of HR and not beyond. Given the majority of studies included in the review involved aerobic exercise, it is not clear whether the findings apply to high intensity resistance training. 

Exercising above 90% of max heart rate has been documented to reduce uterine artery blood flow to 50% of resting values with fetal bradycardia (slow heart rate – indicating fetal distress) occurring (24). Even though fetal heart rate returns to normal post-exercise, it is still unclear what the long-term impact of transient heart rate decelerations on the developing fetus is. 

In summary, until we know more…it makes sense to keep maternal heart rate under 90% while exercising during pregnancy.

OK….BUT IS THERE ANY LINK BETWEEN EXERCISE AND MISCARRIAGE?

There is one Danish study that is frequently cited in the literature that found a link between physical strain and spontaneous abortion (25). Published in 2000, this was an observational study following first time parents. In total, 181 pregnancies were studied. Pregnant women were asked to record their ergonomic and occupational strain. Each morning women were asked to record their previous day’s exposure to physical strain by answering the following question, “How much physical strain did you experience yesterday?”. Strain was ranked from 0 (no strain), 1 (light strain), 2 (moderate strain), to 3 (very high strain). Examples of very high strain included an exhausting tennis match, long-distance running, and frequent lifting of heavy loads. The main finding of this study was that physical strain around the time of implantation (~ 6-9 days after ovulation) was associated with a higher risk of spontaneous abortion – suggesting implantation is a critical period of pregnancy. 

One other very recent study reported a link between high levels of physical activity and early pregnancy loss (pre-visualization on ultrasound) in women who had experienced 1-2 prior miscarriages (26). However, no link was established between exercise and clinically recognized (observed on ultrasound) loss – meaning exercise was somehow associated with miscarriage in early pregnancy but didn’t seem to matter later on. This study supports the notion that implantation is a critical period of pregnancy, particularly for women who have experienced prior loss(es). Further, a 2007 study reported a stepwise link between the amount of exercise – particularly high impact exercise- performed in pregnancy and risk of miscarriage before 18 weeks gestation (27). However - women were asked to report on their exercise behaviours after they were aware they had experienced a miscarriage. In a subgroup analysis including only the women who were asked about their exercise behaviours prior to becoming aware of their miscarriage, the effect sizes were much smaller indicating a significant recall bias. Sidebar: does it surprise you that women who have experienced a miscarriage are more likely to remember their exercise habits differently? Perhaps even blaming themselves for the miscarriage? No shock to me.

SO HOW WOULD PHYSICAL STRAIN CAUSE MISCARRIAGE?

A critical review of the Danish study raises this exact question (28). If we suspect physical strain to affect implantation – how do we explain the lag between implantation (occurring around 6-9 days post ovulation) and spontaneous abortion at 5 weeks gestation (1-2 weeks later)? The authors of the Danish study suggested the lag was due to the high proportion of chromosomal abnormalities seen in early abortions but is physical strain likely to cause mutagenic changes? No. If the theory is that physical strain somehow threatens successful implantation then we need to understand the nature of this threat and through what mechanism this would occur (e.g. hormone changes?). What is it about exercise that could possibly threaten a pregnancy? – We still don’t know. 

But get this…

In direct contrast to the evidence described above, data from two other studies showed physical activity around the time of implantation actually reduced the risk of spontaneous abortion (29-30).

RECENT RESEARCH SAYS NO LINK BETWEEN EXERCISE AND MISCARRIAGE

A 2018 systematic review and meta-analysis of 46 studies reported women who exercised during pregnancy were at no greater risk of miscarriage than those who did not (31). There was no association between volume, intensity, or frequency of exercise and fetal or newborn death. A few limitations to note: the quality of the evidence was ranked as ‘very low’ and the maximum exercise duration included in the review was 60min – so we don’t know for sure whether prolonged (>60min) exercise is the same. Further, in all the studies included in the review exercise was initiated after 8 weeks gestation – precluding the authors from analyzing very early pregnancy loss. 

BUT WHAT DOES IT MEAN THAT THE EVIDENCE WAS ‘VERY LOW’?

It just means that based on the quality and precision of the data that was included in the review, the strength of the evidence is not very high i.e should be interpreted with caution. Usually when there are statistical or adherence related gaps in a study the evidence is considered not as strong as a study that is ‘air tight’ methodologically and quality wise. When you pool several studies together into a ‘meta-analysis’ you are at the mercy of the quality of those studies. 

Although this systematic review gave us a lot of confidence that there is no link between exercise and miscarriage in later pregnancy, we are still uncertain of the association between exercise in early pregnancy and early pregnancy loss. Given the conflicting evidence we cannot say with confidence whether exercise DOES or DOES NOT cause or is any way associated with miscarriage in early pregnancy.

PRACTICAL INTERPRETATION: 

Your age, whether you have experienced prior miscarriage(s), and what (if any) pregnancy complications you are predisposed to/currently experiencing should all be discussed with your healthcare provider and weigh into your decision regarding the type, frequency and intensity of exercise you perform early in pregnancy. Does that completely overwhelm you? Let’s have a chat and we can quell some fears. Email me: [email protected]

IN SUMMARY…

Here is what we know

• Exercise is generally safe and strongly recommended for all women with uncomplicated pregnancies

• All pregnant women without contraindication should engage in a minimum of 150 minutes of moderate-to-vigorous physical activity per week

  • Including both aerobic and resistance training

• Maternal heart rate should not exceed 90% of maximum

• Exercise has important and clinically meaningful benefits to both mom and baby

Here is what we don’t know

• Whether exceeding 90% of maximum heart rate negatively impacts fetal well-being long-term

• Whether high intensity resistance training produces similar transient fetal heart rate decelerations and reductions in placental blood flow as does aerobic training

• Whether implantation is in fact a ‘critical’ period of pregnancy where high impact and/or high intensity exercise should be particularly avoided

• How exercise could cause miscarriage (i.e. via what mechanism?)

   

PRACTICAL IMPLICATIONS:

• Pregnancy is a great time to start exercising if you currently don’t

• If you are active prior to getting pregnancy, it is important you maintain your physical activity throughout pregnancy

• If you have a genetic predisposition to or a current diagnosis of a pregnancy complication it is very important you discuss your exercise plans with your obstetric care provider

• It is normal to be fearful of exercise but that should not prevent you from being active during your pregnancy

• Use the ‘talk test’ to monitor your exercise intensity and exercise within your limitations given your physical activity history and experience level

• Consider working with a certified professional who can guide you safely and effectively during this critical period

I hope you found this article helpful – at the very least you can rest easy knowing that this is an emerging field of research with a lot of great evidence coming out that will help both your obstetric care provider and trainer guide you in the safest and most effective prenatal exercise program. What I hope this article did NOT do is make you more worried to exercise during pregnancy, if that is the case please send me an email and let’s have a conversation to put your mind at ease. There are so many ways to incorporate exercise into your pregnancy. The best prenatal exercise is the type that you feel comfortable doing! If you have any questions about what that should look like for you, send me an email and let’s chat it out.

Well what did you think? Did you learn anything new? Is your attitude toward prenatal exercise the same, worse, better? Drop me a line below, I’d love to hear your thoughts.

References

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