Introduction for the special issue on “exercise during pregnancy”

Exercise is good for you!! That's the consensus with many studies and professional organizations such as the American Heart Association, the American College of Sports Medicine, and the U.S. Department of Health and Human Services strongly supporting that contention for children and adults. But what about exercise for pregnant women? Gone are the times when exercise was frowned upon for pregnant women because it might harm the pregnancy (Downs, Chasan-Taber, Evenson, Leiferman, & Yeo, 2012). Our current enlightened view is that exercise is good for the health of pregnant women and also their children, before and after birth. Certified exercise professionals can even continue to lead moderate to vigorous classes right up to the day of birth with no untoward consequences (Clapp, 2000; Sigurdardottir et al., 2019). In fact, exercise seems to alleviate many of the negative symptoms of pregnancy for the mother from nausea, excessive weight gain, gestational diabetes, to post-partum depression (Hinman, Smith, Quillen, & Smith, 2015; Nagpal & Mottola, 2020). Exercising pregnant moms also seem to have easier births (Perales et al., 2016; Rodríguez-Blanque, Sánchez-García, Sánchez-López, & Aguilar-Cordero, 2019; Wadhwa, Alghadir, & Iqbal, 2020) and smarter, healthier babies (Blaize, Pearson, & Newcomer, 2015; Clapp, 1996; Moyer, Reoyo, & May, 2016). Surprising are the findings that the father can contribute to improving the pregnancy outcomes by exercising prior to conception (Kusuyama, Alves-Wagner, Makarewicz, & Goodyear, 2020). There are intriguing animal studies supporting the benefits of exercise on the immediate and long term pregnancy outcomes for both the mother and offspring. In one such study using transgenic mice that are prone to give birth to offspring with congenital heart defects (CHDs), Dr. Jay and colleagues (Schulkey et al., 2015) showed that voluntary wheel-running activity of older pregnant female mice could reduce the number with CHDs among their offspring. Another study in a mouse model of pregestational diabetes found maternal voluntary wheel running reduced CHDs among their offspring (Saiyin et al., 2019). It is much harder to do such definitive studies in a controlled way on pregnant women and their offspring. In this special issue, we present a broad range of topics relevant to “exercise during pregnancy” including two letters to the editor regarding the guidelines for exercise programs during pregnancy proposed by Barakat (in this issue). The authors have approached this theme in a variety of ways. Dr. May and colleagues conducted two exercise studies (McDonald et al. a and b, in this issue) on one of the most challenging cohorts to study, pregnant women. Unlike rodents, who love to run and voluntarily exercise on a running wheel when given the chance even when pregnant, humans tend to be harder to motivate to exercise. Add to that human tendency, the effects of pregnancy. Despite these hurdles, May and colleagues conducted a prospective, randomized controlled trial study on pregnant women in a region of North Carolina and found that supervised moderate exercise (150 min per week for more than 24 weeks) during pregnancy can have positive effects on the infants 1 month after birth. The control cohort of pregnant women also went to the gym, but had standing or sitting stretching sessions and breathing exercises instead of, as their experimental counterparts did, working out on a treadmill, stationary bike, elliptical machine, rowing and/or stair-stepping equipment. The infants born to the exercised mothers had lower skinfold thickness and percent body fat at 1 month. These results are associated with a reduced risk for obesity for these infants in their later years. This extended and expanded the studies done previously that showed that exercising pregnant women gave birth to neonates with lower birth weights (Davenport et al., 2018; Wiebe, Boulé, Chari, & Davenport, 2015) and suggests a persistent positive effect on the 1 month old infants. Longitudinal studies of these infants into adulthood could be very important to show the positive trajectory continues. The second study (McDonald et al., b in this issue) showed that a similar exercise program also improved the health of the women themselves. For this study, participants were recruited in North Carolina and Spain as part of the ENHANCED by MOM study and GESTAFIT program, respectively. The supervised and monitored aerobically trained women had many blood parameters measured. At mid-pregnancy the aerobic exercise group had better lipid profiles than the controls and at both time points had significantly lower blood insulin levels and less insulin resistance. This is a sign that the metabolic environment in utero was improved and could have a positive impact on the health of the mother as well as on the developing embryo or fetus (Derraik et al., 2020; Yamashita et al., 2014). The strength of these studies was that they did not rely on self-reporting to measure exercise participation, but carefully supervised and monitored the activities. These types of studies are valuable for crystallizing exercise recommendations to optimize the health of pregnant women and their children. Dr. Barakat (in this issue) details a supervised exercise program specifically designed for pregnant women, complete with many pictures. This program recommends various modifications of the exercises to accommodate the changes in the pregnant woman and protect them from stresses and strains throughout their pregnancy (Figure 1). Physiological changes are experienced by pregnant women such as the laxity of joints, alterations in balance as the fetus grows, and breathing rhythms. These changes should be considered in designing an exercise regimen. Exercising under the guidance of exercise professionals with special knowledge of the needs of pregnant women is ideal. However, as indicated in the two letters to the editors by Drs. Ruart and Antoine-Jonville and Mr. Fiutem (in this issue), not everyone has access to this ideal situation for exercising. Socioeconomic status, geography, limited time, and the limited availability of trained experts may preclude attending supervised exercise classes for pregnant women in a gym with the proper equipment. In those cases, minimally supervised exercise or activity such as walking (Figure 2) can be effective in achieving and even surpassing the American College of Obstetricians and Gynecologists (ACOG) recommendation of a minimum of 150 min of moderate exercise per week. The internet and telemedicine could also provide remote supervision, support, and frequent assessment for pregnant women as well. Cardiopulmonary Exercise (CPX) testing is a standard and objective way in which health risk and aerobic capacity is assessed in adults and children. CPX is not commonly employed during pregnancy and pregnancy specific guidance for CPX has not been developed. However, emerging research has suggested CPX is useful in “identifying underlying cardiopulmonary conditions, stratifying the risk of adverse pregnancy outcomes, as well as establishing exercise tolerance/limitation” (Wowdzia and Davenport, in this issue). As research in this area is expected to substantially increase in the coming years, Ms. Wowdzia and Dr. Davenport provide guidance should the CPX test need to be performed on pregnant women. Many measures of physiology change dramatically during the course of a pregnancy. This must be considered and the CPX test must be adapted and interpreted differently for pregnant women. Current guidelines deserve an update with the addition of more detail. For example, resting heart rate increases during a healthy pregnancy. For this reason, heart rates above 99 might not be considered tachycardia for a pregnant woman. Therefore, the cut-off for limiting exercise during pregnancy could be raised from 99 to 120 beats per minute. Extending warm-up and cool-down time is also recommended to compensate for pregnant women having less ability to adjust to starting and stopping exercise. Maximal CPX testing has been conducted on pregnant women during their third trimester. CPX may be valuable when using exercise during pregnancy as a therapy to improve pregnancy outcomes and promote a healthy birth and neonate. CPX is not as widely used as it is in the general population. Authors suggest that more research is needed to have it be more widely accepted for pregnant women. To study the effect of exercise on pregnant women, you need baseline data on the physiology of pregnant women which is hard to come by and the available data are not consistent across studies. Dr. Adamo and colleagues (Dobson et al., in this issue) provide data to fill that gap by following 22 healthy women throughout their pregnancy. Data were captured at early, mid-, and late pregnancy, while the women performed the Submaximal incremental Walking Exercise Test (SWET), a 21-min progressive walking test on a treadmill. As you would expect, physiological parameters changed as the pregnancy proceeded. Overall, heart rate, rating of perceived exertion, and submaximal absolute oxygen uptake were higher in late pregnancy compared to early and/or mid- pregnancy. Physiological/perceptual responses to exercise at late pregnancy were associated with physical activity behaviors measured by accelerometry, gestational weight gain, pre-pregnancy BMI, and age. The authors state “Our findings can be used to inform exercise prescription and design for future PA (physical activity) interventions for pregnant women including the use of submaximal exercise responses as markers of cardiorespiratory fitness.” The authors also attempted to capture physical activity out of the laboratory by asking participants to wear an accelerometer for 7 days while at home. This study was a part of the Physical ACtivity and diEtary implicatioNs Throughout pregnAncy (PLACENTA) Study. There is promising data that exercise can be an effective part of the care that adolescents and adults receive to stop and avoid substance use namely smoking, illicit drug use (opioids, heroin, cocaine), marijuana (cannabis) use, and excessive alcohol consumption (Ashdown-Franks et al., 2020; Dai, Chen, Richardson, & Gordon, 2020). Dr. Adamo and colleagues (Nagpal et al., in this issue) wondered if this tactic could also work on reducing substance use disorders in women before or during pregnancy when the stakes are high for both the mother and the unborn child. It is estimated that “1–5% of pregnancies around the world are affected by substance-use disorders”(Nagpal et al., citing data from NIDA, CCSA, WHO). The clinical recommendation for alcohol use during pregnancy is complete abstinence, as no level of alcohol consumption is known to be safe during pregnancy [recommendation of abstinence from the CDC, AAP, U.S. Surgeon General]. The literature is sparse on this topic of using exercise to control substance abuse in pregnant women, nonetheless the authors found a handful of human and rodent studies that provide encouraging findings. Although the authors found no studies using exercise as an adjuvant therapy option for pregnant substance use disorders, they did find that these women are amenable to engaging in an exercise program during their pregnancy. This suggests that there might be willing subjects for future studies. They also found studies that the use of drugs such as methadone can change the physiological responses of pregnant women to exercise. This finding is important in designing future studies. Animal studies that the authors reviewed did not focus on the effects of exercise on the behavior and health of the pregnant animal, but rather on the behavioral outcomes for offspring which were positive. Thus, adding exercise to the treatment plan for women with substance use disorders before or during pregnancy could help both the mother and the child through many mechanisms. The topic is worthy of further studies. Dr. Volpato and his colleagues (Soares et al., in this special issue) reveal the difficulties in finding the appropriate exercise protocol in animal models to study the effects (and possible risks) of exercise during pregnancy complicated by diabetes. They decreased swimming time compared to previous studies to see if exercise could prevent the effects of the hyperglycemic environment in a milder model of diabetes induced in rats. When neonatal mice are exposed to a chemical isolated from a species of bacterium called streptozotocin, pancreatic beta cells die, but regenerate to some extent, resulting in a milder form of diabetes than when streptozotocin is injected into adult animals (Inhasz Kiss et al., 2013). The swimming protocol with reduced time did not improve parameters in the milder hyperglycemic environment and, in fact, caused some developmental defects in the rat fetuses. Whether this study translates to humans, has not be determined. There are currently specific guidelines being created for women with gestational diabetes (Padayachee, 2015), one of the most common complications of pregnancy that continues to increase in prevalence worldwide. However, establishing the guidelines will require more studies of pregnant women to determine the “type, intensity, and duration of exercise in preventing” gestational diabetes mellitus which is known to negatively impact the health of the pregnant mother and her child (Nasiri-Amiri, Sepidarkish, Shirvani, Habibipour, & Tabari, 2019). Can exercise save the developing brain from effects of alcohol exposure? Milbocker and Klintsova (in this issue) considered the damage that alcohol exposure can inflict on cholinergic neurons and reasoned that behavioral interventions that upregulate cholinergic neurotransmission might help. They remind us that the “mammalian brain cholinergic system is the largest continuous aggregate of neurons in the central nervous system.” The rats exposed to alcohol during a rapid period of brain growth were presented as adults to running wheels that they voluntarily used (unlike most humans) followed by exposure to a complex environment where they could climb and enter all sorts of containers, tubes, and hammocks within a multi-level cage combined with exposure to a changing assortment of colorful “toys” of all shapes and sizes (Gursky & Klintsova, 2017). The wheel running represents aerobic physical exercising while the complex environment could be considered brain exercising that might also enhance activity. While this group's previous findings showed promise that this combination of interventions termed “super interventions” improves many other outcomes of alcohol exposure, it was not reflected in normalizing the microanatomical parameters of the cholinergic neurons in the nucleus basalis of Meynert (NBM), the source of cortical cholinergic input, and prefrontal cortex (PFC) in adult rat brains. The authors declare no potential conflict of interest. Data sharing is not applicable to this article as no new data were created or analyzed in this study.