This chapter should be cited as follows:
Khurana R, Mackillop L, et al, Glob. libr. women's med.,
ISSN: 1756-2228; DOI 10.3843/GLOWM.413653

The Continuous Textbook of Women’s Medicine Series – Obstetrics Module

Volume 8

Maternal medical health and disorders in pregnancy

Volume Editor: Dr Kenneth K Chen, Alpert Medical School of Brown University, USA Originating Editor: Professor Sandra Lowe

Chapter

Sport and Exercise

First published: February 2021

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INTRODUCTION

Pregnancy is a period of rapid and profound physiological adaptation.¹ As pregnancy progresses, estrogen, progesterone and placental hormone levels rise within the maternal circulation and affect virtually every system of the woman’s body to support the needs of the growing fetus. Despite the benefits of exercise, many women are reluctant to be physically active due to fatigue, feeling unwell or fear for the safety of their fetus.

Exercise is established to be a critical component of health across the lifespan, yet, only recently have we begun to realize the powerful influence of prenatal exercise to optimize the health of two generations. Guidelines around the world recommend that women without contraindications, be physically active throughout pregnancy based on extensive literature demonstrating the safety and health benefits of exercise for both mother and fetus.^{2,3,4,5,6,7,8,9,10,11,12,13} Yet, only approximately 15% of women are sufficiently active to meet current guidelines and derive potential health benefits during pregnancy.¹⁴ The goal of this review is to examine physiological adaptations to pregnancy, discuss the various evidence-based recommendations regarding physical activity in pregnancy and to summarize the data regarding prevention and management of pregnancy and delivery complications. We also discuss recommendations regarding participation in organized or elite sport during pregnancy.

ANATOMICAL AND PHYSIOLOGICAL ADAPTATIONS TO PREGNANCY

There are profound maternal physiological adaptations during pregnancy that allow for growth and nourishment of the rapidly developing fetus and prepare the mother for the physiological challenge of birth.

Cardiovascular system

Total body water increases progressively during pregnancy due primarily to renal sodium retention. As a result, plasma volume increases by approximately 50% by around 32 weeks' gestation. Plasma expansion is offset by progesterone-mediated vasodilatation leading to decreased systemic vascular resistance. Maternal cardiac output starts to rise as early as 10 weeks' gestation and reaches a plateau in the early third trimester at 30–50% above the non-pregnant values.¹⁵ This is a consequence of an increase in both stroke volume and heart rate, the latter of which increases steadily through pregnancy to around 10–15 bpm higher than baseline by term.¹⁵ Diastolic, and to a lesser extent systolic, blood pressure is subject to a modest fall in the first half of pregnancy, increasing to non-pregnant levels by term.

Respiratory system

There is a 20% increased oxygen demand during pregnancy which leads to profound respiratory physiological adaptation. There is an increase in minute ventilation due to the effect of progesterone on respiratory centers in the hypothalamus. Counterintuitively, respiratory rate is unaffected by pregnancy, but tidal volume increases. Many women feel these changes and experience physiological breathlessness of pregnancy.¹⁶

Consequences for exercise

Pregnant women have enhanced vasodilation which when combined with a greater amount of blood flow going to the skin when exercising in the heat can increase the risk of fainting. Pregnant women may need to drink more to stay hydrated to support this increased volume as well as to offset increased insensible losses from skin and increased minute ventilation.

When pregnant women exercise, there is a further increase in oxygen and cardiac demand, and the cardiovascular and respiratory response naturally increase. This can lead to a significant rise in heart rate and respiratory rate, particularly in the third trimester, at a lower exercise threshold than in the non-pregnant woman.¹⁶ Hyperemia of the nasal mucosa causes nasal congestion, which can make nasal breathing more difficult and further increase the sensation of breathlessness.

Blood pressure and cardiac output may be affected by maternal posture. In late pregnancy, the gravid uterus may mechanically obstruct the aorta and vena cava in the supine position leading to hypotension.^17,18,19 Cardiac output may be enhanced if a left or right lateral tilt is maintained in the recumbent position. Therefore, care of posture during exercise at later stages of pregnancy is important.

Metabolism

Major changes in bone metabolism occur to meet the demands of the growing fetus. Intestinal calcium absorption increases due in part to increased production of 1,25-dihydroxyvitamin D (1,25(OH)2D). Calcium is also released from the maternal skeleton due to increased bone resorption stimulated by production of parathyroid hormone (PTH) related protein (PTHrP) by breast and placenta. This results in loss of bone from the maternal skeleton during pregnancy which continues until lactation ceases and then recovers. There is evidence that maintaining, particularly weight-bearing, physical activity in pregnancy attenuates this bone loss.²⁰

Posture/gait

There are many biomechanical adaptations during pregnancy. Hormonal changes from increased levels of progesterone and relaxin cause ligamentous laxity. The effects of the enlarging uterus and weight gain contribute to a shift in the center of gravity anteriorly. There is also loss of foot arch height and rigidity which does not recover after pregnancy.²¹ There is an increasing lumbar lordosis as well as decreased stability at the hip and knee. Later in pregnancy, women may be unable to see where they place their feet due to the growing abdomen and there is some evidence of significantly reduced ankle proprioception in the third trimester.²² These changes lead to difficulties with balance. Indeed, there appears to be an increased falling risk for pregnant women, particularly in the third trimester.^23,24 To compensate for these changes, there is decreased step length and increased step width with slower walking velocity, and longer time on both feet (double-support time), adaptation that progresses with gestation.^25,26,27 

Although relaxin is produced to soften the ligaments of the pelvis to allow for delivery of the baby, it acts globally and all joints can be affected. A characteristic trait of pregnancy is that women appear to become more flexible as a result of looser joints. Thus, women are advised to avoid overstretching which could harm the muscle, and avoid activities that are jarring or have quick turns which may increase the risk of injury. Overall, it is especially important that pregnant women have an adequate warm up and cool down to minimize the risk of injury.

HISTORICAL AND GLOBAL PERSPECTIVE ON GUIDELINES FOR EXERCISE DURING PREGNANCY

Throughout history, physical activity during pregnancy has been strongly influenced by our cultural beliefs and the status of women within our society; only recently have our recommendations been shaped by scientific inquiry. For centuries occupational labor was the most common form of physical activity and was dictated by one’s social class. As early as 1400BC in the book of Exodus, it was noted that the labor and delivery of sedentary mistresses was more difficult than those of slave women who worked throughout pregnancy.²⁸ In the late 19th century, pregnant women were encouraged to remain indoors and keep themselves “confined” as it was thought that strenuous activity would lead to a “dislodged” uterus or harm to the fetus.²⁹ Support for this notion grew as some of the first published scientific studies demonstrated that resting in a maternity home prior to birth was positively associated with increased infant birth weight³⁰ and led to the fear that physical activity, especially in late pregnancy was detrimental to neonatal weight.

By the 1920s and 30s, significantly more women entered the workforce due to the First World War and there was evidence that women in the workplace experienced fewer pregnancy complications.³¹ The first ever set of guidelines for exercise during pregnancy was published by the American College of Obstetricians and Gynecologists (ACOG) in 1985.³² This guideline was based on the consensus opinion of a panel of obstetricians and endorsed the safety of most aerobic activities during pregnancy. The second country to produce a guideline for physical activity in pregnancy was Spain in 1993³³ followed by Canada, Norway, Japan, Australia, France, Denmark and the United Kingdom.^{28,34,35,36,31,37,38,39,40,41} Several countries have updated their guidelines in the past four years to reflect our current understanding of the impact of prenatal exercise on maternal/fetal health. The SOGC/CSEP 2019 Canadian Guideline for Physical Activity throughout Pregnancy, the 2017 UK Chief Medical Officers/Royal College of Obstetricians and Gynaecologists (RCOG), 2016 Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG) and the 2019 US Guideline all recommend that pregnant women engage in at least 150 minutes of moderate-intensity physical activity over the course of the week,^{42,43,44,45,46,47} while the 2015 ACOG Guideline encourages 20–30 minutes of moderate intensity physical activity on most, if not all days of the week.⁴⁸ Incorporating a variety of aerobic and resistance training is encouraged to achieve greater health benefits. In addition, the Canadian Guideline now recommends pelvic floor muscle training (e.g., Kegel exercises) may be performed on a daily basis to reduce the risk of urinary incontinence; however, instruction in proper technique is recommended to obtain optimal benefits. These guidelines are based on up-to-date evidence base which strongly demonstrates the safety and benefits of exercise during pregnancy for pregnant women without a medical reason not to exercise (contraindication).^{2,3,4,5,6,7,8,9,10,11,12,49}

Over the course of two thousand years, we have come full circle in our view of prenatal physical activity. The latest global recommendations for prenatal exercise represent a fundamental shift in our view of prenatal physical activity away from focusing on the potential risks of exercise towards an effective prescription to improve health, focusing on the risks of not being physically active during pregnancy. However, we are far from empowering women from all socioeconomic and cultural backgrounds to engage in physical activity throughout their pregnancy. Lack of time, financial cost, social support, work and family duties as well as cultural practices continue to prevent many women from taking part in physical activity. Co-ordinated efforts from clinicians, researchers and policy makers will be needed to enhance access to physical activity and remove barriers for pregnant women.

CONTRAINDICATIONS TO PRENATAL PHYSICAL ACTIVITY

Although the majority of pregnant women will benefit from engaging in moderate intensity physical activity, there are certain medical conditions where exercise may adversely impact the health of the mother and/or fetus. These conditions or contraindications are classified as either absolute or relative. In the event that an absolute contraindication develops, strenuous physical activity is not recommended, but women may continue activities of daily living as directed by their healthcare professional.^43,46,47 In contrast, the development of relative contraindications warrants a discussion between the pregnant patient and her obstetric healthcare provider to determine the potential risks and benefits of physical activity during pregnancy. Currently, there is discrepancy between guidelines across the world regarding the classification of specific conditions to relative versus absolute contraindications. This is due, at least in part, to the extremely limited empirical evidence and abundance of caution supporting these contraindications. All pregnant women are encouraged to speak with their obstetric healthcare provider about any questions they may have regarding specific contraindications.

Table 1 compares the contraindications listed by the 2019 Canadian guidelines, 2015 ACOG Committee Opinion and the 2016 RANZCOG guidelines. Of note, the 2017 UK RCOG guidelines and the 2018 US Guideline do not list specific contraindications for physical activity and are intended for healthy women with uncomplicated pregnancies. Generally, there is a high degree of agreement for the absolute contraindications (the RANZCOG guidelines do not list contraindications as absolute or relative). Although there is a lack of data on exercise in complicated pregnancies, restricting physical activity in women with ruptured membranes, preterm labor, vaginal bleeding and incompetent cervix seems very sensible. Additionally, conditions that are associated with placental insufficiency such as growth restriction or those in which maternal disease may lead to decreased oxygen carrying capacity/low cardiac output (cardiac, pulmonary, anemia) have the potential to compromise the fetus as the mother shunts blood flow and oxygen away from the uterus to exercising muscle. Some of the relative contraindications deserve further comment. Uncontrolled diabetes is listed as a contraindication by many guidelines. Although exercise can improve hyperglycemia, it increases the risk for hypoglycemia. During pregnancy, diabetic women are often managed with “tight” control and are potentially at greater risk of hypoglycemia. They should be given specific instructions on management to avoid and treat hypoglycemia prior to, during and after exercise (see Table 2). Although orthopedic limitations may prevent the use of certain types of exercise in pregnancy, other modalities (such as swimming), may be tolerated. The ACOG guidelines list “being extremely sedentary” as a contraindication. However, more recent guidelines recommend exercise even in previously inactive women with the recommendation that they start gradually.^43,46,47 As well, it is clear that women with obesity may be at higher risk for complications that exercise can prevent (such as gestational diabetes and pre-eclampsia) and therefore there is no reason to prevent these women from exercising as long as they do not have any other contraindications.⁶ There is concern about exercise in women who are heavy smokers as smoking interferes with fetal oxygenation because of competitive binding of fetal hemoglobin by carbon monoxide. Indeed, analyses of cohort data identified that women who smoked and exercised during pregnancy had a greater risk of fetal death compared to non-smoking women who did not exercise.⁵⁰

It is important to note that a contraindication to prenatal exercise does not mean that there is a recommendation for bed rest. Bed rest continues to be prescribed by many clinicians,⁵² despite known negative impacts on maternal health such as anxiety, depression, muscle atrophy, bone loss, venous thromboembolism and gestational diabetes.⁵³However, the evidence for improvement in pregnancy outcomes is sparse.^{54,55,56,57,58,59} A recent systematic review and meta-analysis of randomized controlled trials of bed rest in complicated pregnancies⁶⁰ found that in developed countries, bed rest actually doubled the risk of very premature birth. In developing countries, there appeared to be a slight gain in birth weight, but these results may not be generalizable as the studies in developing countries were all performed in Zimbabwe in the 1980s and 90s.

CONSIDERATIONS FOR IMPLEMENTATION

Pregnant women are encouraged to be physically active throughout pregnancy; however, there are a number of adaptations and safety concerns to consider. While some of these considerations are based on empirical data most of this guidance is based on the expert opinion of the guideline consensus panels developing the recommendations.

Safety precautions

There are several safety precautions that should be undertaken when exercising while pregnant (Table 3). Pregnant women are advised to avoid engaging in physical activity in excessive heat, especially with high humidity as it increases the likelihood of becoming dehydrated. As dehydration is associated with a reduced ability to regulate body heat, pregnant women who exercise in the heat are theoretically at increased risk of overheating. Unfortunately, all of the research on the impact of maternal exercise on core temperature to date has been conducted in thermoneutral conditions.^10,61 As such, there is no empirical information available regarding thermoregulation of pregnant women during exercise in hot or humid environments. Until safety information is available, it is recommended to avoid this type of exercise. Instead, exercise is recommended to be performed under cooler conditions such as in air conditioning or by avoiding the midday heat by exercising in the shade or in the early morning or evening.

Another important safety consideration is that pregnant women avoid activities that risk trauma to the belly, especially in the second and third trimester as the uterus rises out of the pelvis. This includes activities with a high risk of physical contact or falling such as horseback riding, downhill skiing, riding a non-stationary bike, playing soccer, hockey or other sports. This recommendation is included for a number of reasons. First, the primary concern is that direct trauma to the fetus may cause harm. Although there is some protection of the fetus through the uterus and amniotic fluid, direct trauma to the abdomen can cause harm to the fetus through loss of amniotic fluid, placental abruption and premature labor. We also know that pregnant women are twice as likely to fall as non-pregnant women (see posture/gait section). Pregnant women often feel more fatigued, and have looser ligaments and tendons due to the effects of relaxin which can further increase the risk of falling. There is also theoretical concern about the shear force which occurs when the body stops before the internal organs and fetus. This potentially carries a risk of placental abruption. Finally, even the most experienced skier or cyclist only has limited control over their surrounding environment or their kicking fetus.

Scuba diving is not recommended during pregnancy as there is no known safe depth for the fetus. Scuba divers breathe compressed air which contains nitrogen; as the diver swims deeper, more nitrogen gas enters the tissues of the body. As a diver rises to the surface, the nitrogen is removed from the body via the lungs, which are not functioning in the fetus in utero. As a result, bubbles of nitrogen may form in the fetal circulation which can cause tissue and nerve damage.

High altitude is defined as any elevation above 2500 m or 8200 feet. In non-pregnant populations there are well known physiological adaptations to exercise at altitude including a reduced aerobic capacity but also increased ventilation, heart rate and metabolic rate. However, the combined impact of altitude, exercise and pregnancy above 2500 m is unknown in lowlanders or women who were born below 2500 m. In the few altitude studies investigating prenatal exercise at altitudes up to 2500 m, exercise was tolerated but some potential adverse responses of the fetus were observed. It is important to note that this recommendation does not extend to women born and raised at altitude. Indeed, a recent case study was published describing the physical activity levels of a third trimester native highlander during ascent from 3440 m to Everest Base Camp in Nepal at 5360 m. Incredibly she engaged in nearly 300 minutes of moderate to vigorous physical activity each day with no apparent maternal, fetal or neonatal complications.⁶² However, further research is critically needed in this area.

Reasons to stop physical activity

When prescribing exercise during pregnancy it is important to reinforce that there are a number of reasons to stop physical activity and consult with a healthcare provider. These conditions can represent the initiation of labor or be a symptom of a clinically relevant complication (see Table 4).

Sedentary behaviors

Sedentary behaviors are activities that expend less than 1.5 metabolic equivalents in the seated, reclined or supine position.⁶³ These behaviors are distinct from a lack of physical activity; however, the literature often uses them interchangeably. In non-pregnant populations, approximately 55–60% of the waking day is spent in sedentary behaviors. Engaging in high amounts of sedentary behavior has been linked to an increased risk for type 2 diabetes, cardiovascular disease and mortality.⁶⁴ Although the impact of sedentary behavior in non-pregnant adults and children is well known, the impact of sedentary behavior on maternal and fetal health is not well understood. A recent systematic review of the literature found that pregnant women spend 57–78% of their time in sedentary behaviors⁶⁵ and time spent in sedentary behaviors increase as pregnancy progresses.⁶⁶ This review identified that sedentary behaviors were significantly higher among women who delivered macrosomic infants, although the association with gestational diabetes did not reach statistical significance.⁶⁵ The interaction between sedentary behavior and pregnancy outcomes remains a relatively new area of research and in the coming years, we expect this to be an area for further investigation.

HOW TO START BEING ACTIVE DURING PREGNANCY?

Women who were not active or had low levels of physical activity prior to pregnancy are encouraged to be physically active in pregnancy to derive clinically meaningful health benefits. However, a gradual progression towards the recommended minimum 150 minutes of moderate intensity exercise is recommended to reduce the risk of injury or excessive fatigue. Recently, dose–response relationships were identified whereby increasing frequency/intensity/volume of exercise was associated with a greater reduction in the odds of developing gestational diabetes, pre-eclampsia, gestational hypertension, excessive gestational weight gain and depressive symptoms.^6,11,12 Since an important goal is to be physically active for at least 150 minutes per week, there are many ways to engage in physical activity. To increase step counts, it is important to give pregnant women ideas of how to increase the number of steps they take each day. One way is to suggest that she get off a few bus stops earlier, park farther away from her destination or take the stairs instead of the elevator to add extra steps to her day. Pregnant women can rake leaves, cut the grass, garden, and even shovel snow (but be aware of lower back and technique). However, many studies of physical activity in pregnant women focused on the health benefits of walking, which is an activity that is both accessible and inexpensive for most pregnant women. Of course, playing with children keeps women active and eliminates one of the barriers to being physically active. It is important to note that even physical activity below the recommended guidelines was associated with some health benefits; some activity is better than none! However, more physical activity was associated with greater health benefits.

EXERCISE IN THE ATHLETE

In 2017/2018 the International Olympic Committee produced five documents which reviewed available literature regarding pregnant athletes.^{67,68,69,70,71} Unfortunately, this is an incredibly understudied population and there is minimal information regarding the safety and benefits on high intensity, long duration or extreme volumes of exercise. One study in six pregnant Olympic-level athletes identified fetal bradycardia and high umbilical artery pulsatility index during a period of strenuous exercise.⁷² However, the clinical significance of this is unclear as both measures normalized quickly after cessation of exercise with no apparent adverse impact on the fetus.

The recommendations for physical activity in pregnancy tend to be cautious, which may not serve an athlete well. As a result, the IOC Consensus Panel was unable to provide concrete recommendations about the safety limits of exercise for pregnant athletes. It was recommended that women who continue to engage in elite sport and/or exercise substantially beyond current guidelines be closely monitored by their obstetric healthcare provider to monitor fetal growth and development, and the potential development of contraindications to exercise. Maintaining an open conversation between the pregnant woman, her coaches, trainers and healthcare providers is critical since the safety of high level training during pregnancy has not been established. However, it is equally important to consider the potential mental health issues that may result by limiting the activity of an athlete who is used to training for multiple hours each day, as well as the possible detraining effects that may occur. A few things to keep in mind when dealing with the pregnant athlete include that there is no research on the relationship between weight gain and fetal growth in athletes. As such, the 2009 Institute of Medicine guidelines for weight gain in pregnancy are typically used.⁷³ The frequency of eating disorders is higher in athletes (20–22%) than non-athletes (3–9%), especially in certain sports and providers caring for these patients may need to be consider disordered eating when dealing with a pregnant woman who has inadequate weight gain. Although light to moderate resistance training appears to be safe in healthy pregnant women, data are lacking on the heavy weight training undertaken by some elite athletes. There are concerns about Valsalva during weight training as well as harming the pelvic floor with high weights.

Pregnancy in an athlete may bring up medical, legal, ethical and human rights issues. Due to concerns about pregnancy discrimination, athletes may be reluctant to disclose that they are pregnant to sporting bodies, coaches, team members, etc. Of paramount importance should be the health of the woman and her fetus, and limits placed upon participation and activity should be determined in consultation with a qualified medical practitioner. The rights of the athlete to autonomy and privacy need to be balanced with the rights of sporting bodies to protect themselves from potential liabilities. Institutions and sporting bodies should ensure that they have appropriate insurance coverage, and work to end pregnancy discrimination to ensure the mental and physiological health of their athletes. Several bodies have established guidelines to deal with these issues in pregnant athletes including the Australian Sports Commission⁷⁴ and the National Collegiate Athletic Association.⁷⁵ Additional research is critically needed to appropriately counsel pregnant athletes.

IMPACT OF PHYSICAL ACTIVITY DURING PREGNANCY ON HEALTH OUTCOMES

Although the majority of women have healthy pregnancies, some will develop complications that may adversely impact the mother and/or fetus. However, prenatal physical activity is emerging as a simple, low cost therapy to reduce the risk of developing some pregnancy complications. In the next section we describe the impact of physical activity during pregnancy on common pregnancy complications.

Gestational diabetes mellitus

Gestational diabetes mellitus (GDM) is a common disorder that increases the risks for pregnancy complications (see chapter on GDM). Although GDM typically resolves after delivery, women with a history of GDM have a significantly increased risk of developing type 2 diabetes (RR 7.43), and the fetus may be at lifelong elevated risk for developing obesity and diabetes.^76,77

In non-pregnant individuals, exercise is a core therapy to prevent and manage diabetes. It has also been found to prevent GDM in pregnancy. A recent meta-analysis (26 studies, n = 6934) examining the impact of exercise on the prevention of GDM found that women who exercised during pregnancy had a 38% reduction in the odds of developing GDM compared to women who did not. Dose–response analysis of the exercise-only randomized controlled trials identified that a greater volume of physical activity was associated with a greater reduction in the risk of developing GDM.⁶ Indeed, 591 MET minutes per week (equivalent to 137 minutes per week of moderate intensity exercise) was associated with a 25% reduction in the odds of developing GDM.

Exercise is also beneficial for women diagnosed with diabetes. A single bout of exercise results in a decline in circulating blood glucose values.⁸ The greater the volume of exercise (defined as the intensity of the exercise multiplied by the duration), the greater the reduction in blood sugars. However, even just walking for 10–15 minutes results in a reduction in blood glucose.⁸ This response was identified in both women with and without diabetes; however, the reduction in blood glucose was greater in women with diabetes usually because they started at a higher pre-exercise value.⁸ With chronic exercise, mean fasting blood glucose at the end of pregnancy was 2.71 mmol/L lower in women with diabetes who exercised compared with women who did not.⁸ Although there were limited number of studies reporting on this, in women with diabetes the number of women taking insulin was not different between groups; however, the amount of insulin used was lower in women who exercised.⁸ In our experience, some women are able to use short bouts of walking after meals and avoid the need for insulin or metformin. Importantly, the incidence of hypoglycemia following an acute bout of exercise was low. However, it is still very important to educate pregnant women with diabetes on symptoms of hypoglycemia when prescribing exercise to them.

Hypertensive disorders of pregnancy

Hypertensive disorders of pregnancy include gestational hypertension and pre-eclampsia, and affect approximately 10% of women and cause significant morbidity for mother and fetus.⁷⁸ Although both disorders typically resolve after delivery they are both independent risk factors for future cardiovascular disease ranging from a 70% increased risk with gestational hypertension to a 440% increased risk in women who developed severe pre-eclampsia with fetal death.⁷⁹ The data are now so strong that in 2011 the American Heart Association listed pregnancy complications including history of pre-eclampsia, gestational diabetes or gestational hypertension as risk factors, as strong as smoking for the future development of cardiovascular disease.⁸⁰

Exercise is a cornerstone for prevention and treatment of hypertension in general populations; however, the effectiveness of exercise in prevention during pregnancy is poorly understood. A recent meta-analysis examining the impact of prenatal exercise on pregnancy outcomes found that the odds of developing gestational hypertension was reduced by 39% and pre-eclampsia by 41% in women who exercised compared to women who did not. In a dose–response analysis, it was identified that 260 MET minutes per week (equivalent to 60 minutes per week of moderate intensity exercise) was associated with a 25% reduction in the odds of developing pre-eclampsia, and 401 MET minutes per week (93 minutes per week of moderate intensity exercise) for gestational hypertension.⁶ Currently, the mechanisms underlying the prevention of pre-eclampsia with prenatal exercise are not well understood. However, animal studies have suggested that exercise may promote the growth and efficiency of the placenta, improvements in vascular function and a reduction in oxidative stress and inflammation.⁸¹

Weight gain

Weight gain is a healthy and normal adaptation to pregnancy (covered in the chapter on weight gain during pregnancy). Studies have demonstrated that women who gain above recommendations have an increased risk of developing GDM and hypertensive disorders of pregnancy, cesarean delivery, and having large for gestational age baby. Women who gain below the recommended weight gain guideline have an increased risk of having a small for gestational age baby and preterm birth. Importantly, gaining below recommendations is a greater contribution to having a small baby than other important risk factors such as smoking during pregnancy or being classified as underweight prior to pregnancy.⁸²

A recent meta-analysis (15 studies, n = 3519 women) demonstrated that prenatal exercise reduced the risk of having excessive weight gain during pregnancy by 32%.¹¹ However, the risk of inadequate gestational weight gain was also increased by 32%.¹¹ Despite this, exercise seems to reduce the risk of large for gestational age infants without increasing the risk of small for gestational age (see section on fetal outcomes).

Depression and depressive symptoms

Depression affects approximately 13% of pregnant women and strongly predicts the development of postpartum depression (see chapter on Mental Health During Pregnancy).^83,84 A recent meta-analysis of prenatal exercise interventions identified that exercise was associated with a 67% reduction in the odds of developing depression during pregnancy (5 studies and n = 683) and a reduction in depressive symptoms (19 studies and n = 3316).¹² Dose–response analysis identified a moderate effect size in the reduction in depressive symptoms is achieved with 644 MET minutes per week (150 minutes per week of moderate intensity exercise).

Urinary incontinence

According to the International Continence Society, urinary incontinence (UI) is “a complaint of any involuntary leakage of urine” and represents a common problem that deeply impacts quality of life for women.⁸⁵ Up to 75% of all pregnant women will develop UI during pregnancy, and approximately one-third persist into the postpartum period.⁸⁶ This condition is known to have significant, detrimental effects on physical activity, travel, social relationships, and emotional health. Pelvic floor muscle training or “kegels” have been used to strengthen the pelvic floor, which will better support the organs of the pelvis, including the bladder, bowel and uterus. However, the best evidence for pelvic floor training in reducing urinary incontinence is under the guidance of a pelvic floor specialist, which many women may have difficulty accessing. During pregnancy, pelvic floor muscle training alone or in combination with other forms of exercise reduces the odds of prenatal urinary incontinence by 50%, and the odds of postpartum urinary incontinence by 37% (24 studies n = 15,982).⁵ However, the studies reviewed in these meta-analyses included training in proper technique by a pelvic health specialist.

Impact of exercise on labor and delivery

The impact of prenatal exercise on labor and delivery outcomes including cesarean section, instrumental delivery (forceps or vacuum) and length of labor are not well understood. A recent review of the literature suggested that women who exercised during pregnancy had a 24% reduction in the odds of having an instrumental delivery, but there was no difference in the odds of having a cesarean delivery between women who did and did not exercise during pregnancy.⁷

Fetal outcomes

Although exercise is associated with substantial benefits, only 15% of women are sufficiently active to derive health benefits.¹⁴ Although there are a number of barriers to prenatal exercise, concern for the health of the fetus has been suggested to be a primary reason for inactivity. Many women believe that exercise can increase the risk of early pregnancy loss. In 2007, data from the Danish National Birth Cohort included 93,000 women who were interviewed about their physical activity in early pregnancy.⁸⁷ This study found that women who engaged in high volume or high impact exercise equivalent to 7 hours per week of running in the first trimester had an increased risk of miscarriage. However, in this cohort, two-thirds of women were interviewed after having a miscarriage. In 2014 the authors followed up on this study and restricted their analysis to women who were interviewed about exercise before they had miscarried because of concerns about recall bias about exercise in the women who were interviewed after a miscarriage.⁸⁸ In this study, they did not find that exercise increased the risk of having a miscarriage. These data are supported by a systematic review which did not identify evidence of increased risk of a miscarriage with prenatal exercise.²

Another concern of prenatal exercise is that it might increase the risk of having a preterm delivery (<37 weeks gestation). Worldwide, 1 in 10 babies are born premature which increases their likelihood of being admitted to the neonatal intensive care unit (NICU) and complications such as respiratory distress syndrome, intraventricular hemorrhage and necrotizing enterocolitis.⁸⁹ In the long term, prematurity is associated with an increased risk of cognitive impairment, developmental delays and chronic health issues. However, meta-analyzed evidence from exercise only randomized controlled trials (27 studies and n = 5283) found no increased risk of prematurity with exercise.⁴

Neonates born growth-restricted (SGA or IUGR) are more likely to require admission to the NICU, and suffer from respiratory and metabolic disorders in infancy.^90,91 Early studies suggested exercise may result in a decrease in birth weight.^92,93,94,95 It was believed that exercise would redistribute blood flow, oxygen and nutrients to the working muscle away from the fetus resulting in restricted growth. In contrast, babies born large for gestational age (LGA) are more likely to be delivered by cesarean section, experience shoulder dystocia during delivery, and have hypoglycemia or jaundice.^96,97 A review of 45 cohort/case–control studies demonstrated that being born large (>4000 g) increased the risk of overweight and obesity during childhood by 66% (OR 1.66; 95% CI 1.55–1.77).⁹⁸ Reducing the risk of having a small or large for gestational age baby optimizes the lifelong health of the baby. Recent work has demonstrated that prenatal exercise normalizes birthweight by not increasing the risk of having a small newborn and by reducing the risk of having a large infant by 39%.⁴ In addition to reducing the risk of short-term complications during and following delivery, this may have long-term implications for the health of the baby. However, long-term follow up of exercise interventions are needed.

FUTURE WORK

Studies over the past 50 years have strongly demonstrated the safety and benefits of engaging in physical activity during pregnancy. However, there remain a number of critical gaps regarding our understanding of prenatal exercise. The current body of literature is generally confined to studies examining the impact of physical activity within the scope of guidelines for exercise during pregnancy and we subsequently have a poor understanding of the limits of physical activity. Specifically, there is limited information regarding the potential benefits or harms for both mother and fetus of high intensity (i.e., near maximal), long duration (>1 hour) and high volumes of exercise to guide women wishing to engage in activities substantially exceeding current recommendations. There also remains a paucity of evidence regarding the impact of specific types of exercise that many women wish to continue during pregnancy including resistance training, yoga, pilates, core strengthening, among others. Nearly all studies are limited to aerobic activities with an overwhelming majority examining the impact of walking, cycling or swimming on health outcomes. Finally, research is urgently needed in specific subgroups of women including those with relative contraindications (e.g., recurrent pregnancy loss, previous spontaneous pre-term delivery, twin pregnancy), as well as in those in whom there is good theoretical evidence that prenatal exercise might be particularly beneficial (e.g., GDM, type 1 and type 2 diabetes mellitus, women with a pre-pregnancy BMI ≥25.0 kg/m²).

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