Pregnancy and Exercise in Mountain Travelers

Abstract

Pregnant women are traveling to high altitude and evidence-based recommendations are needed. Yet, there are limited data regarding the safety of short-term prenatal high-altitude exposure. There are benefits to prenatal exercise and may be benefits to altitude exposure. Studies evaluating maternofetal responses to exercise at altitude found the only complication was transient fetal bradycardia, a finding of questionable significance. There are no published cases of acute mountain sickness in pregnant women, and data suggesting an increase in preterm labor are of poor quality. Current recommendations across professional societies are overly cautious and inconsistent. Non-evidence-based restrictions to altitude exposure can have negative consequences for a pregnant women's physical, social, mental, and economic health. Available data suggest that risks of prenatal travel to altitude are low. Altitude exposure is likely safe for women with uncomplicated pregnancies. We do not recommend absolute restrictions to high altitude exposure, but rather caution and close self-monitoring.

In Brief

In uncomplicated pregnancies, short visits to high altitude are probably safe, and restricting travel and activities may have unnecessary negative consequences.

Introduction

Limited data exist on the safety of travel and exercise during short-term sojourns to high altitude (generally defined as elevation greater than 2500 m above sea level) during pregnancy. Nonetheless, understanding the risks and potential benefits to short-term high-altitude exposure during pregnancy is critical. Pregnant women are traveling to and recreating at high altitude regardless of recommendations or a lack thereof. In a survey of tourists in high altitude areas of Summit County, about 3% of respondents were pregnant (¹), and in a more recent survey of active women, nearly half had been to high altitude at some point during their pregnancy (²). Simply assuming that high altitude travel and mountain activities are dangerous for pregnant women is non-evidenced-based practice and may be ignoring important benefits that high altitude exposure may provide to the pregnant woman. We know that exercise in pregnancy has benefits, including a higher incidence of vaginal delivery and lower incidences of excessive gestational weight gain, gestational diabetes, gestational hypertensive disorders, preterm birth, and low birth weight (³).

Caution, careful consideration, and more research are needed for pregnant people traveling to high altitude. There are well-documented risks to low-altitude dwellers residing at high altitude for their entire pregnancy, including preeclampsia, eclampsia, hypertension of pregnancy, placenta previa, placental abruption, and preterm labor (^4–11). Fetal complications also are increased in this population, including small for gestational age, intrauterine growth retardation (IUGR), neonatal respiratory distress, pulmonary hypertension, and stillbirths but not congenital malformations (^4,5,7,9–11). Furthermore, in high-altitude dwellers, infant birth weight decreases with altitude, and lower birth weight is associated with greater health complications in infancy and beyond (^9,12). These maternal and fetal complications are less likely to occur in women from genetically adapted native high-altitude populations, such as Tibetans and Andeans (¹³). The discussion that follows applies to low altitude residents from nongenetically adapted populations engaging in short-term travel and activity at high altitude.

Evidence for Safety of High Altitude Exercise

It is unknown exactly at what stage during pregnancy high altitude residents develop maternal and fetal complications, nor how long a pregnant person needs to be exposed to high altitude during the prenatal period to put her at risk. Very few studies have evaluated exercise in pregnant women in hypobaric hypoxia. Three older articles have assessed maternal and fetal responses to exercise after short visits (a few hours) at elevations lower than most Colorado ski resorts (^14–16) and below the typical threshold for the development of high altitude illness (¹⁷) (Table). These studies are further limited by the small number of women included, a total of 25, all of whom were otherwise sedentary (^14–16). Among these 25 women, the only complication observed after 3 min of exercise was transient fetal bradycardia in one fetus in each study. In two studies, the participant with the fetal bradycardia was a woman who smoked (^14,15). Transient fetal bradycardia also has been noted when pregnant women exercise above 90% of their maximal heart rate but has not demonstrated any known impact on neonatal outcomes (¹⁸). Whether this transient fetal bradycardia has any long-term significance is unclear.

Table - Studies on Exercise and Pregnancy at High Altitude

	Altitude (m)	N	Maternal Effects	Fetal Effects
Baumann, 1985	2228	12	↑HR with altitude and exercise ↑RR, BP with exercise Mild contractions after ascent - No ↑ with exercise	11/12 normal tracings 1 brady after exercise in smoker
Baumann, 1986	2200	6		5/6 normal tracings 1 brady after exercise in smoker
Artal, 1995	1830	7	↓ Aerobic capacity ↑ CO, SV No change in glucose, lactate, cathecols 1 woman with contractions requiring observation	5/6 normal tracings 1 brady after exercise

Current Guidelines

Based on this extremely limited data, many organizations have adopted guidelines about pregnancy and high altitude. Citing the articles discussed above, the International Climbing and Mountaineering Federation offers the following contraindications to altitude travel after 20 wk gestation: chronic or pregnancy-induced hypertension (without specific mention of preeclampsia), ultrasound diagnosed impaired placental function, intrauterine growth restriction, maternal heart or lung disease, anemia and suggests that smoking could be a contraindication (¹⁹). It is unclear how the 20-wk date was chosen as a cutoff. The types of maternal heart or lung disease are not specified, though in fact, patients with well-controlled asthma tolerate high altitude travel up to 6000 m without difficulties (²⁰) and some consider moderate and high altitude beneficial to asthma (^21,22).

The 2020 American College of Obstetrics and Gynecology Committee Opinion on Exercise in Pregnancy, citing the Artal article, states that “women living at sea level were able to tolerate physical activity up to altitudes of 6000 ft, suggesting this altitude is safe in pregnancy [Atral, 1995], although more research is needed,” and does not make any further recommendations (²³). This Opinion updates the 2009 version where the same Artal article was cited with the statement that “engaging in physical activity at higher altitudes carries various risks” (²⁴). The current Society of Obstetricians and Gynecologists of Canada and Canadian Society for Exercise Physiology guideline on exercise in pregnancy, however, makes the following recommendation: lowlander women (i.e., living below 2500 m) should avoid physical activity at high altitude (>2500 m) (²⁵). Evidence is lacking for such a recommendation, and would imply that a pregnant woman should not travel to or engage in any activity (including walking) in many commonly visited North American mountain resort areas.

An Alternative Perspective

We appreciate the grave consequences associated with poor pregnancy outcomes, but there are downsides to being overly cautious, too. It is important to recognize that restricting exposure to altitude may inadvertently lead to other activity restrictions. Activity restrictions have negative impacts on pregnant women and their fetuses. Such restrictions have been associated with trabecular bone loss, low birth weight, venous thromboembolism, and deconditioning. Decondition, in particular, has downstream negative impacts on metabolism, appetite, and sleep cycles (²⁶). Activity restrictions may even increase the risk of preterm labor (²⁷). There also is a potential negative economic impact for professional athletes or others who depend on participation in mountain sports occurring at altitude for their income (for example, climbing or ski instructors). In addition, restricting altitude exposure may result in social isolation as pregnant women may then not be able to recreate with friends or attend social or professional events occurring at altitude.

For decades, women were counseled to avoid exercise in pregnancy and recommendations focused on absolute and relative contraindications (²⁴). Recent recommendations, however, recognize the harms of activity restrictions, as well as the many benefits of exercise to both mothers and babies (²³). There may be other benefits to traveling and exercising at high altitude during pregnancy that have not yet been identified. One of these potential benefits may be improved fitness. Women who continue exercising at a high level during their pregnancy can maintain and even increase their V˙O₂ max postpartum (¹⁸). This relates to altitude exposure in that exercise at altitude is known to increase V˙O₂ max (²⁸). Thus, restricting altitude exposure can put pregnant athletes who might otherwise train at altitude at a disadvantage (¹⁸).

In addition to the physical and physiological benefits of exercise during pregnancy, exposure to nature, such as in the mountains, may be associated with better pregnancy outcomes (²⁹). In addition, nature exposure has documented mental health benefits (³⁰) and better mental health also is associated with better pregnancy outcomes (^31,32). Greater study is needed to understand the interplay between the potential risks of short-term hypoxic exposure at high altitude and the physical and psychological benefits of mountain activities during pregnancy.

What Are Pregnant Women Really Doing?

It also is worth noting that pregnant women are not following the cited guidelines. A quick search of any social media platform will identify pregnant women engaging in high altitude activities, such as skiing, snowboarding, and climbing. A 2016 study on participation in outdoor activities during pregnancy surveyed 300 women, almost half of whom had been to high altitude at some time during their pregnancy. This cohort included 28 who had been above 4000 m, one who summited a 7000-m peak and two who sky-dived during pregnancy (²). In this group of women, overall maternal and fetal complication rates were low, were similar between those who had traveled to high altitude during pregnancy and those who had not, and were similar, or lower, to what is seen in the general U.S. population (²).

In this large survey, first trimester bleeding was more common in women who had not gone to high altitude, perhaps because this occurrence made them worried about travel later in pregnancy (²). A physiologic explanation for this finding is unclear. The other difference between groups was that preterm labor was more common in women with altitude exposure during pregnancy (²). Anecdotally, obstetricians from Colorado high altitude communities report preterm labor as the most common complication they see in pregnant visitors (³³). This finding likely warrants further study. Although there was more preterm labor in women exposed to high altitude at some point during pregnancy, there was no difference in any neonatal complications except the need for supplemental oxygen, a complication that did not translate into greater neonatal intensive care admissions or other longer-term concerns (²). Of course, this single survey has several limitations — some of the participants were recalling pregnancies more than 20 plus years before, complications were not verified with official medical records and the survey was targeted to active women in geographical areas where they would have access to high altitude. Despite these limitations, the results of this survey are reassuring for healthy active pregnant women, and highlight the need for high-quality prospective studies.

What Are the Real Risks?

There is potential for negative impact in restricting exposure to altitude, as this also may result in limiting pregnant women’s participation in mountain sports. Unfortunately, many of these activities, including skiing and climbing, come with their own set of non-evidence-based, caution-based recommendations. A review of prenatal exercise guidelines across seven countries found that pregnant women are frequently warned about the risks of sports while pregnant, and yet there are no data to suggest an increased risk of harm (³⁴).

There is no scientific evidence that mountain sport activities, such as skiing or climbing, are indeed dangerous during pregnancy, and while we recognize the potential for trauma in these sports, the limited available evidence may suggest the contrary. A German study by Drastig et al. (³⁵) surveyed 32 experienced women climbers using a self-reported online questionnaire and found that the rate of injury was low and similar to nonpregnant women, and that climbing did not significantly increase the risk of preterm birth. Half of the women surveyed climbed until 36 wk gestation, demonstrating the potential for pregnant women to participate in this activity throughout the duration of their pregnancy.

The potential for trauma due to falls is the primary concern with recreational activities such as, climbing or skiing, while pregnant. While abdominal trauma during early pregnancy is unlikely to cause fetal or uterine harm, the risk of injury to the fetus increases in the second and third trimester after the uterus becomes an abdominal organ. Even when engaging in “lower risk” activities, such as walking, falls are the most common cause of trauma hospitalizations in pregnant women (³⁶). Of course, falls that occur while climbing or skiing have potential to be of higher consequence due to higher speeds or fall distances. Pelvic fractures in pregnant women, while exceedingly rare, can lead to placental abruption, the disruption of the attachment between the placenta and the uterus. Abruption carries high potential for serious consequences with fetal mortality ranging from 10% to 47% (³⁷). While direct trauma to the abdomen can cause placental abruption, this also may occur from a sudden deceleration similar to that which occurs during a motor vehicle accident due to shearing forces between the different tissue properties of the rigid placenta and more elastic and muscular uterus. Such injury could theoretically occur with a climbing fall when caught by the belayer if the pregnant climber or the pregnant belayer is wearing a traditional harness rather than a full body harness. We have not found such a case documented in the literature. The risk of exposure to altitude that accompanies these sports has already been discussed.

It is important to note that many of the risks of trauma are easily mitigated, and women are willing to do so. The pregnant climbers in the Drastig survey were highly experienced, on average having climbed for 9 years prior to their pregnancy, and 90% of respondents willingly modified their climbing behavior by climbing less difficult routes and ceasing to lead climb in later pregnancy (³⁵). Also, while the published data did not report the use of full body harnesses, presumably this modification would have been made by nearly all women, as it quickly becomes difficult to fit a traditional harness properly over a gravid uterus. Such adaptations may be more appropriate in managing risk than complete restrictions. Also, while we recognize that this data is limited, it suggests that climbing may not pose a significant risk to pregnant women, and we have no actual data to support the contrary. A similar line of reasoning could be applied to other sports, such as skiing and mountaineering.

Another potential risk of high-altitude activities is that of acute mountain sickness (AMS), a syndrome occurring in unacclimatized individuals traveling to altitudes above 2500 m. The diagnosis is made clinically when an individual experiences a headache plus one of the following: fatigue or weakness, dizziness or lightheadedness, or gastrointestinal symptoms, including nausea vomiting or anorexia] (³⁸). Humans adjust to the hypobaric hypoxia of altitude through the process of acclimatization. This occurs through several mechanisms involving almost every organ system, but the most important changes in the first 24 h are cardiopulmonary (¹⁷). Heart rate and ventilation increase. If the body does not acclimatize due to too rapid ascent or physiologic predisposition, the traveler is at risk for AMS. Many of the normal physiologic changes of pregnancy mimic that of acclimatization. Heart rate, cardiac output, and ventilation all rise, along with an increase in 2,3-diphosphoglyceric acid (2,3-DPG). This increased 2,3-DPG shifts the hemoglobin binding curve to the right, making it easier to unload oxygen from hemoglobin and, thus, increasing oxygen delivery to the tissue, including the fetus. Pregnant women have a more brisk ventilatory acclimatization and have a higher resting PaO₂ and lower pCO₂ than their nonpregnant counterparts at high altitude (³⁹). Given these physiologic changes, in theory, AMS should be rare in pregnant women. We found no published cases. Certainly, there is no evidence that pregnant women are more at risk for AMS. Importantly, acetazolamide, the recommended medicine for pharmacologic prophylaxis of AMS (⁴⁰), is teratogenic and should be avoided in pregnancy.

As discussed, data are lacking to support or refute the safety of short-term sojourns to high altitude and participation of pregnant women in mountain sports. There exists, however, some physiological evidence to suggest that the hypoxia of high altitude does not put the fetus at risk for hypoxia. In a study done in Peru, researchers compared samples of maternal blood and fetal scalp blood from women in labor in Lima at sea level and in Cerro de Pasco at 4000 m (⁴¹). Although average maternal PaO₂ was markedly lower at 4000 m compared with sea level (61 mm Hg vs 91 mm Hg), fetal PaO2 was nearly identical (19 mm Hg vs 21 mm Hg) (⁴¹). These findings suggest that the fetus is relatively protected from the maternal hypoxic stresses of high altitude, though these measurements have not been repeated in exercising pregnant women.

Conclusions

We recommend a more practical and nuanced approach and suggest that pregnant women with uncomplicated pregnancies travel to high altitude with caution and close self-monitoring. For most healthy women with uncomplicated pregnancies, short term travel to high altitude is probably safe and restricting travel to and activities at high altitude may have unnecessary negative consequences. We recommend prudence and that women take additional time to acclimatize before exercising. For women with any preexisting pregnancy complications, we recommend against adding the additional stress of a hypoxic environment, especially given the potential lack of easy access to medical care, and counsel against high altitude travel. For women with underlying medical conditions that might impair oxygen transport and acclimatization (for example, severe anemia, heart conditions, non-asthma lung disease), we recommend extreme caution and discussion with their care providers on a case-by-case basis. Finally, we call for prospective studies to better understand the risks and benefits of mountain travel and activity during pregnancy.