Recent advances in understanding maternal and fetal responses to exercise

Perspectives of Midwives and Nurse Practitioners in Kentucky on Exercise Counseling During Pregnancy: A Qualitative Study

Introduction

Healthcare professionals are in an optimal position to deliver exercise information to pregnant women, yet previous research suggests this seldom happens. Midwives and nurse practitioners, who may have more time with pregnant women, are particularly well suited for this role.

Objectives

This qualitative study examined the exercise advice and counseling provided by midwives and nurse practitioners in Kentucky, focusing on the barriers they face.

Methods

Twenty-one midwives and nurse practitioners were recruited until the sample size reached saturation. A survey with open-ended questions was distributed to potential participants in regional hospitals, universities, and professional associations. The framework method was employed to identify common themes in the responses from participants.

Results

Five main themes emerged: nature of advice, discussing exercise benefits, safety concerns, barriers to counseling, and suggestions for improvement. Findings revealed that midwives and nurse practitioners recommend moderate exercise, aiming for 150 min weekly, monitoring heart rate for intensity, continuing prepregnancy exercise routines, starting low-intensity exercise during pregnancy such as walking, slowing down as pregnancy advances, and avoiding heavy lifting and vigorous activities. Many midwives and nurse practitioners in our sample took a reactive approach to exercise counseling, providing exercise advice if pregnant women asked questions or if they were at high risk for hypokinetic diseases. It was also observed that midwives and nurse practitioners discussed the maternal benefits of exercise more than the fetal benefits. Only a few nurse practitioners and midwives were content with their counseling, while the majority did not feel their counseling was effective.

Conclusions

Many midwives and nurse practitioners in Kentucky provided comprehensive and accurate physical activity guidelines to pregnant women. However, there is room for them to improve: proactive counseling should include discussions on fetal benefits and using the "talk test" for exercise intensity. Advising patients to slow down as pregnancy progresses should be reconsidered, and evidence-based guidance on specific exercises should be prioritized.

Rating of perceived exertion as a tool for managing exercise intensity during pregnancy: a scoping review

OBJECTIVE

During pregnancy, international guidelines recommend ≥150 min of moderate-intensity aerobic physical activity per week, with an intensity perceived as fairly light to somewhat hard on the Borg Rating of Perceived Exertion (RPE) scale (ranging from 6 'no exertion' to 20 'maximal exertion', corresponding to 60% to 80% of maximum heart rate). However, the determination and monitoring of exercise intensity seem to be a particular source of confusion, and the most effective method to monitor exercise intensity remains uncertain. This study aimed to examine existing research on the correlation between the Borg RPE scale and maternal heart rate (MHR) for monitoring exercise intensity during pregnancy.

DESIGN

Scoping review using the mixed methods appraisal tool (MMAT) and the Grading of Recommendations, Assessment, Development and Evaluations (GRADE) approach.

DATA SOURCES

PubMed, Web of Science and SPORTDiscus were searched from 16-17 April 2023, with a subsequent search on 1 November 2023.

ELIGIBILITY CRITERIA

We included studies investigating the correlation between perceived intensity on the Borg RPE scale (6-20) and MHR during exercise in pregnant individuals and written in English/Scandinavian language. There were no restrictions on publication year or study design.

DATA EXTRACTION AND SYNTHESIS

Two independent reviewers screened the articles based on title and abstract. Selected articles were read in full text and reference lists of screened articles were also checked. Out of 120 studies screened, six articles met the inclusion criteria after removing one duplicate. The results were qualitatively summarised to provide an overview of common themes and variations between studies. MMAT and GRADE assessed the risk of bias and the certainty of the evidence.

RESULTS

The six studies involved a total of 260 healthy pregnant individuals (gestational week: from 16 to 38), with various exercise protocols (cycling, walking, running and resistance exercise) and intensities (from light to moderate). Three studies supported the Borg RPE scale to estimate exercise intensity during pregnancy, while three found no correlation between this scale and MHR. The certainty of the evidence was graded as low to moderate, with a potential risk of bias due to small sample sizes, incomplete outcome data and inconsistencies across studies.

CONCLUSION

The mixed results highlight the complexity of monitoring exercise intensity during pregnancy. Using both the Borg RPE scale and MHR might be better than using them separately for monitoring exercise intensity during pregnancy. Due to limited and inconsistent research, more extensive studies are needed.

Cool mama: Temperature regulation during high-intensity interval running in pregnant elite and recreational athletes

Background

Regular exercise during pregnancy is beneficial, but athletes often exceed the recommended 150 min of moderate-intensity activity, incorporate high-intensity exercises. The upper limit for exercise intensity and duration on fetal and maternal safety remains uncertain. A concern is a maternal core body temperature of >39.0 °C, potentially increase the risk of heat-related fetal malformations and complications during pregnancy. Blood flow redirection for thermoregulation could compromise fetal cardiovascular function, increasing the risk of miscarriage and preterm labor. This study evaluated whether pregnant women (gestational weeks 25-35) were at risk of exceeding a core body temperature of 39.0 °C during high-intensity running. We also investigated effects on skin temperature, fluid loss, and thermal sensation, comparing pregnant athletes to non-pregnant controls.

Methods

In this comparative cross-sectional study, 30 elite and recreational athletes (pregnant n = 15) completed up to five high-intensity treadmill-intervals. Core and skin temperature were continuously measured. Body weight was utilized to calculate the amount of fluid loss.

Results

Highest core body temperature were 38.76 °C and 39.56 °C in one pregnant and non-pregnant participant, respectively. Pregnant participants had lower core body temperatures (mean difference -0.47 °C, p ≤ 0.001) initially and a smaller increase (0.10 °C, p ≤ 0.003) during later intervals compared with the non-pregnant controls. Pregnant participants also showed a greater increase in skin temperature (4.08 ± 0.72 °C vs. 3.25 ± 0.86 °C, p = 0.008) and fluid loss (0.81 ± 0.19 L vs. 0.50 ± 0.12 L, p˂0.001).

Conclusion

Physiological changes in pregnancy may enhance thermoregulation, indicating that high-intensity interval runs are unlikely to pose a risk of exceeding a core body temperature of 39 °C for pregnant athletes.

Sex Differences in Energy Metabolism: A Female-Oriented Discussion

The purpose of this review is to delineate aspects of energy metabolism at rest and during exercise that may be subject to sex differences and the potential underlying mechanisms involved. It focuses on distinct aspects of female physiology with an oriented discussion following the reproductive life stages of healthy, eumenorrheic females, including premenopausal time frames, pregnancy, perimenopause, and menopause. Finally, this review aims to address methodological challenges surrounding sexual dimorphism in energy metabolism investigations and confounding factors in this field. During resting conditions, females tend to have higher rates of non-oxidative free fatty acid clearance, which could contribute to lower respiratory exchange ratio measures. At the same time, carbohydrate energy metabolism findings are mixed. In general, females favor lipid energy metabolism during moderate-intensity exercise, while men favor carbohydrate energy metabolism. Factors such as age, dietary intake, genetics, and methodological decisions confound study findings, including properly identifying and reporting the menstrual cycle phase when female subjects are eumenorrheic. Pregnancy presents a unique shift in physiological systems, including energy metabolism, which can be observed at rest and during exercise. Changes in body composition and hormonal levels during the post-menopausal period directly impact energy metabolism, specifically lipid metabolism. This change in physiological state factors into the evidence showing a reduction in our understanding of sex differences in lipid metabolism during exercise in older adults. This review reveals a need for a focused understanding of female energy metabolism that could help exercise and nutrition professionals optimize female health and performance across the lifespan.

Circulating long-chain acylcarnitine concentrations are not affected by exercise training in pregnant women with obesity

The purpose of this study was to determine the effect of exercise during pregnancy in sedentary women with obesity on longitudinal changes in long-chain acylcarnitine (LC-AC) concentrations. We hypothesized that exercise training would significantly decrease circulating LC-ACs throughout gestation compared with a nonexercise control group. Pregnant women with obesity considered otherwise healthy [n = 80, means ± SD; body mass index (BMI): 36.9 ± 5.7 kg/m²] were randomized into an exercise (n = 40, aerobic/resistance 3 times/wk, ∼13th gestation week until birth) or a nonexercise control (n = 40) group. At gestation week 12.2 ± 0.5 and 36.0 ± 0.4, a submaximal exercise test was conducted, and indirect calorimetry was used to measure relative resting energy expenditure (REE), as well as respiratory exchange ratio (RER) at rest. Fasting blood samples were collected and analyzed for LC-AC concentrations. Fitness improved with prenatal exercise training; however, exercise training did not affect circulating LC-AC. When groups were collapsed, LC-ACs decreased during gestation (combined groups, P < 0.001), whereas REE (kcal/kg/day, P = 0.008) increased. However, average REE relative to fat-free mass (FFM) (kcal/kg FFM/day) and RER did not change. There was an inverse relationship between the change in RER and all LC-ACs (except C18:2) throughout gestation (C14: r = -0.26, P = 0.04; C16: r = -0.27, P = 0.03; C18:1: r = -0.28, P = 0.02). In summary, a moderate-intensity exercise intervention during pregnancy in women with obesity did not alter LC-ACs concentrations versus control, indicating that the balance between long-chain fatty acid availability and oxidation neither improved nor worsened with an exercise intervention.NEW & NOTEWORTHY This research showed that a moderate-intensity prenatal exercise program, consisting of aerobic and resistance training, did not negatively impact normal alterations in substrate supply and demand for the mother and the offspring throughout gestation. Findings provide support for metabolic safety of exercise during pregnancy.

Monitoring Physical Activity Intensity During Pregnancy

For apparently healthy pregnant women, regular physical activity is recommended. The American College of Obstetricians and Gynecologists (ACOG) created recommendations for physical activity and exercise during pregnancy in 1985. At that time, pregnant women were advised to not exceed a heart rate of 140 beats per minute with physical activity. The heart rate recommendation was subsequently removed with the recommendations published in 1994, 2002, and 2015. In 2020, the ACOG updated its recommendations on physical activity for pregnant and postpartum women. The recommendation included exercising at a "fairly light to somewhat hard" perceived intensity and at less than 60-80% of age-predicted maximum heart rate, usually not exceeding a heart rate of 140 beats per minute. Women often seek advice from healthcare providers on physical activity during pregnancy, yet providers report concern about giving appropriate physical activity guidance. This paper summarizes the key scientific literature on monitoring absolute and relative exercise intensity in relation to the current ACOG recommendations, providing background on intensity-related concepts used in the recommendation. This paper also provides practical guidance to assist healthcare providers in relaying this information to pregnant women.

Substrate metabolism during exercise: sexual dimorphism and women's specificities

The aim of this review is to discuss sexual dimorphism of energy metabolism, and to describe the impact of women's hormonal status on substrate oxidation during exercise. Many evidences indicate that sex steroids play a pivotal role in the sex-related differences of body composition and energy substrate storage. Compared with men, women rely more on fat and less on carbohydrates at the same relative exercise intensity. Scientific data suggest that 17-β oestradiol is a key hormone for the regulation of body composition and substrate metabolism. However, in women, measurements with stable isotopic tracers did not highlight any difference in whole-body substrate oxidation rates between the follicular and luteal phases of the menstrual cycle during endurance exercise. The remaining discrepancies about the effect of menstrual cycle-related hormone fluctuations on substrate oxidation could be partly explained by the exercise intensity, which is an important regulator of substrate oxidation. Due to their specific nature and concentration, the synthetic ovarian hormones contained in oral contraceptives also influence substrate metabolism during endurance exercise. However, more studies are needed to confirm that oral contraceptives increase lipolytic activity during endurance exercise without any substantial (or detectable) effect on substrate utilization. Pregnancy and menopause also modify body composition and substrate utilization during exercise through specific hormonal fluctuations.This review highlights that the hormonal status is likely to affect substrate oxidation during exercise in women emphasizing the need to take it into consideration to optimize their health and performance.

Exercise as a Therapeutic Intervention in Gestational Diabetes Mellitus

Gestational Diabetes Mellitus (GDM) is defined as any degree of glucose intolerance with onset or first recognition during pregnancy. Regular exercise is important for a healthy pregnancy and can lower the risk of developing GDM. For women with GDM, exercise is safe and can affect the pregnancy outcomes beneficially. A single exercise bout increases skeletal muscle glucose uptake, minimizing hyperglycemia. Regular exercise training promotes mitochondrial biogenesis, improves oxidative capacity, enhances insulin sensitivity and vascular function, and reduces systemic inflammation. Exercise may also aid in lowering the insulin dose in insulin-treated pregnant women. Despite these benefits, women with GDM are usually inactive or have poor participation in exercise training. Attractive individualized exercise programs that will increase adherence and result in optimal maternal and offspring benefits are needed. However, as women with GDM have a unique physiology, more attention is required during exercise prescription. This review (i) summarizes the cardiovascular and metabolic adaptations due to pregnancy and outlines the mechanisms through which exercise can improve glycemic control and overall health in insulin resistance states, (ii) presents the pathophysiological alterations induced by GDM that affect exercise responses, and (iii) highlights cardinal points of an exercise program for women with GDM.

Applying the Ecological Model to understand pregnant women's perspectives on the modifiable constraints to physical activity during pregnancy: A qualitative research study

The benefits of physical activity (PA) during pregnancy are widely reported; however, PA practice is seemingly not a valued habit among pregnant women attending public antenatal health centres in the Eastern Cape of South Africa. Guided by the ecological model, we sought to explore modifiable barriers to PA among pregnant women.Semi-structured interviews involved a purposive sample of 15 pregnant women. Interview questions were guided by the ecological model constructs at intrapersonal, interpersonal, and environmental level. Thematic analysis was applied to summarise the modifiable barriers to PA during pregnancy.Three main themes emerged, based on the modifiable barriers to PA during pregnancy that belong to the varying Ecological Model constructs. On the intrapersonal level, 5 themes emerged, namely, time-constraint beliefs, feeling of tiredness, low energy, lack of motivation, and a lack of knowledge on benefits and types of PA. Two themes emerged for the interpersonal level, lack of PA advice and lack of information on PA recommendations and guidelines. Another theme defined the environmental level lacking resources. Most themes related to individual factors, which prevent PA-promoting behaviour.Overall, intrapersonal factors relating to tiredness and exhaustion, lack of time beliefs, work and household commitments, and lack of motivation were key modifiable barriers to PA by the women. The findings provide insights into possible interventional strategies to optimise PA during pregnancy among women in this setting. Appropriate knowledge, education and advice on the benefits, types, and intensity of PA in pregnancy are needed.

Physical activity and gestational weight gain predict physiological and perceptual responses to exercise during pregnancy

BACKGROUND

Exercise is known to improve the health of the pregnant woman and her child. Studies that have evaluated physiological parameters during prenatal exercise have conflicting results. Better understanding of these physiological responses can modify exercise prescriptions, safety, and monitoring strategies. We examined the association between age, prepregnancy body mass index (BMI), gestational weight gain (GWG), and physical activity (PA) levels, factors that may influence a change in physiological (HR, VO₂ responses) and perceptual (RPE) responses to acute exercise throughout pregnancy.

METHODS

Twenty-two healthy pregnant women (31.4 ± 3.7 years) performed a Submaximal incremental Walking Exercise Test (SWET). Early- (13-18 weeks), mid- (24-28 weeks), and late-pregnancy (34-37 weeks) were compared. VO₂ (L/min; ml/kg/min), HR (bpm), and RPE were collected at the end of each test stage. PA was determined by accelerometry. We associated PA levels, GWG, prepregnancy BMI, and age with HR, RPE, and VO₂ responses.

RESULTS

HR, RPE, and absolute VO₂ were higher in late-pregnancy compared to earlier time points (p < .05; η² = 0.299-0.525). Regression models were built for HR (all time points), RPE (early- and late-pregnancy), and VO₂ (L/min; late-pregnancy). HR (late-pregnancy) was predicted by time in vigorous PA, GWG, age, and prepregnancy BMI (r² = 0.645; SEE = 5.84). RPE (late-pregnancy) was predicted by sedentary time, GWG, prepregnancy BMI, and age (r² = 0.662; SEE = 1.21).

CONCLUSION

Physiological/perceptual responses were higher in late-pregnancy compared to other time points and associated with combined PA, GWG, prepregnancy BMI, and age. These findings can be used to modify exercise prescriptions and designs for future PA interventions in pregnant women.

Physical Activity and Exercise During Pregnancy and the Postpartum Period: ACOG Committee Opinion, Number 804

Exercise, defined as physical activity consisting of planned, structured, and repetitive bodily movements done to improve one or more components of physical fitness, is an essential element of a healthy lifestyle, and obstetrician-gynecologists and other obstetric care providers should encourage their patients to continue or to commence exercise as an important component of optimal health. Women who habitually engaged in vigorous-intensity aerobic activity or who were physically active before pregnancy can continue these activities during pregnancy and the postpartum period. Observational studies of women who exercise during pregnancy have shown benefits such as decreased gestational diabetes mellitus, cesarean birth and operative vaginal delivery, and postpartum recovery time. Physical activity also can be an essential factor in the prevention of depressive disorders of women in the postpartum period. Physical activity and exercise in pregnancy are associated with minimal risks and have been shown to benefit most women, although some modification to exercise routines may be necessary because of normal anatomic and physiologic changes and fetal requirements. In the absence of obstetric or medical complications or contraindications, physical activity in pregnancy is safe and desirable, and pregnant women should be encouraged to continue or to initiate safe physical activities. This document has been revised to incorporate recent evidence regarding the benefits and risks of physical activity and exercise during pregnancy and the postpartum period.

Physical activity and incidence of subclinical and clinical pregnancy loss: a secondary analysis in the effects of aspirin in gestation and reproduction randomized trial

OBJECTIVE

To estimate the association between physical activity and risk of subclinical and clinical pregnancy loss among women with a history of pregnancy loss.

DESIGN

Prospective cohort study as a secondary analysis of the Effects of Aspirin in Gestation and Reproduction randomized controlled trial of preconception-initiated low-dose aspirin among women with one or two prior pregnancy losses.

SETTING

Four U.S. clinical centers, 2007-2011.

PATIENT(S)

Women with confirmed pregnancy (n = 785) as determined from hCG testing in longitudinally collected biospecimens.

MAIN OUTCOME MEASURE(S)

Subclinical loss of pregnancy detected only by hCG testing and clinically recognized loss.

RESULT(S)

Among 785 women (mean [SD] age, 28.7 [4.6] years) with an hCG-confirmed pregnancy, 188 (23.9%) experienced pregnancy loss. In multivariable models adjusted for confounders, compared with the first tertile of physical activity (median = 7.7 metabolic equivalent of task hours/week), there was a roughly twofold higher risk of subclinical loss in the second (risk ratio = 2.06; 95% confidence interval, 1.03-4.14) and third tertiles (risk ratio = 1.92; 95% confidence interval, 0.94-3.90), with median metabolic equivalent of task hours/week of 27.8 and 95.7, respectively. No relations were observed between physical activity and clinically recognized loss.

CONCLUSION(S)

Risk related to physical activity is different for pregnancy failure close to the time of implantation compared with that for later, clinical pregnancy loss. Higher physical activity levels were associated with an elevated risk of subclinical loss (i.e., pregnancies detected only by hCG, n = 55); however, no relationship was observed with clinically recognized loss. Further work is required to confirm these findings, assess generalizability to women without prior losses, and evaluate mechanisms.

ETHICAL APPROVAL

Each participating center's Institutional Review Board approved the study, and participants provided written informed consent. The trial was registered on ClinicalTrials.gov (NCT00467363), and a Data Safety and Monitoring Board provided oversight.

Maternal Responses to Aerobic Exercise in Pregnancy

Exercise is one of the most physiologically challenging stressors requiring the coordination of metabolic, respiratory, and cardiovascular responses to meet increased energy requirements of the working muscle. During pregnancy, all women without contraindication are encouraged to exercise as part of a healthy lifestyle. Pregnancy itself is associated with profound physiological adaptations to the maternal cardiovascular, respiratory, and metabolic systems, which serve to support the needs of the growing fetus. Therefore the physiological adaptations to exercise during pregnancy are more pronounced and critically important. This review provides an overview of our current understanding of the physiological adaptations to acute prenatal exercise.

Exercise guidelines for gestational diabetes mellitus

The prevalence of gestational diabetes mellitus (GDM) is increasing worldwide. This disease has many detrimental consequences for the woman, the unborn foetus and child. The management of GDM aims to mediate the effects of hyperglycaemia by controlling blood glucose levels. Along with pharmacology and dietary interventions, exercise has a powerful potential to assist with blood glucose control. Due to the uncertainty of risks and benefits of exercise during pregnancy, women tend to avoid exercise. However, under adequate supervision exercise is both safe and beneficial in the treatment of GDM. Therefore it is vital that exercise is incorporated into the continuum of care for women with GDM. Medical doctors should be able to refer to competently informed exercise professionals to aid in GDM treatment. It is important that exercise treatment is informed by research. Hence, the development of evidence-based guidelines is important to inform practice. Currently there are no guidelines for exercise in GDM. This review aims to assess the efficacy of exercise for the management of GDM in order to establish an exercise prescription guideline specific to the condition. It is recommended that women with GDM should do both aerobic and resistance exercise at a moderate intensity, a minimum of three times a week for 30-60 min each time.

Exercise in pregnancies complicated by obesity: achieving benefits and overcoming barriers

An increasing number of women are entering pregnancy in an overweight or obese state. Obese women and their offspring are at increased risk of adverse perinatal outcomes, which may be improved by regular moderate-intensity antenatal exercise. Current guidelines recommend that all pregnant women without contraindications engage in ≥30 minutes of moderate-intensity exercise on a daily basis. However, obese women are usually less physically active and tend to further reduce activity levels during pregnancy. This commentary summarizes the potential short- and long-term benefits of antenatal exercise in obese pregnant women, highlights the challenges they face, and discusses means of improving their exercise levels. In addition, we make recommendations on exercise prescription for pregnancies complicated by obesity.

Strenuous exercise during pregnancy: is there a limit?

OBJECTIVE

The purpose of this study was to evaluate fetal responses to strenuous exercise in physically active and inactive women.

STUDY DESIGN

Forty-five healthy women (15 who were nonexercisers, 15 who were regularly active, 15 who were highly active) underwent a peak treadmill test at 28 weeks' gestation to 32 weeks 6 days' gestation. Fetal well-being (umbilical artery Doppler indices, fetal heart tracing/rate, biophysical profile [BPP]) was evaluated before and after exercise. Uterine artery Doppler scans were also obtained.

RESULTS

Umbilical and uterine artery Doppler indices were similar among activity groups and did not change with exercise (P > .05). BPP and fetal heart tracings were reassuring in all groups. However, subgroup analyses showed transient fetal heart rate decelerations after exercise and elevated umbilical and uterine artery Doppler indices in 5 highly active women. After this, BPP and fetal heart tracings were reassuring.

CONCLUSION

Overall fetal well-being is reassuring after short-duration, strenuous exercise in both active and inactive pregnant women. A subset of highly active women experienced transient fetal heart rate decelerations and Doppler changes immediately after exercise. Athletes may push beyond a threshold intensity at which fetal well-being may be compromised. However, potential impact on neonatal outcomes is unknown.

Towards an Understanding of Change in Physical Activity from Pregnancy Through Postpartum

OBJECTIVE: The purpose of this paper was to describe the rationale, data collection, and proposed analyses for examination of mediators of change in physical activity from pregnancy to postpartum among a cohort of pregnant women. METHOD: The Pregnancy Infection and Nutrition 3 (PIN3) Study enrolled 2006 pregnant women into the cohort from 2001 to 2005. All women lived in central North Carolina upon enrollment. Physical activity was assessed using a self-reported one week recall, measured twice during pregnancy and once each at 3- and 12-months postpartum. On a subset of women, one-week accelerometer measures were also collected during the two postpartum time periods. Potential mediators (intrapersonal, interpersonal, community) were collected during pregnancy and postpartum through interviews and take home questionnaires. RESULTS: To assess mediation of physical activity among our cohort, we will first describe change in physical activity and the mediators, as well as their associations, through pregnancy into the postpartum period. Following this, the product of coefficients approach will be applied to examine whether each measure had indirect effects on change in physical activity. Each individual level mediator will be examined one at a time and across the time points in which it was available. The Sobel standard error approximation formula will be used to test for significance of the mediation effect. CONCLUSIONS: This study will provide evidence to develop appropriate interventions targeted at physical activity and will help focus efforts on the appropriate time periods between pregnancy and postpartum.

Type of delivery is not affected by light resistance and toning exercise training during pregnancy: a randomized controlled trial

OBJECTIVE

We examined the effect of light-intensity resistance exercise training that is performed during the second and third trimester of pregnancy by previously sedentary and healthy women on the type of delivery and on the dilation, expulsion, and childbirth time.

STUDY DESIGN

We randomly assigned 160 sedentary women to either a training (n=80) or a control (n=80) group. We recorded several maternal and newborn characteristics, the type of delivery (normal, instrumental, or cesarean), and dilation, expulsion, and childbirth time.

RESULTS

The percentage of women who had normal, instrumental, or cesarean delivery was similar in the training (70.8%, 13.9%, and 15.3%, respectively) and control (71.4%, 12.9%, and 15.7%, respectively) groups. The mean dilation, expulsion, and childbirth time did not differ between groups.

CONCLUSION

Light-intensity resistance training that is performed over the second and third trimester of pregnancy does not affect the type of delivery.

Physical activity patterns during pregnancy

PURPOSE

The aim of the study was to describe the mode, frequency, duration, and intensity of physical activity among pregnant women, to explore whether these women reached the recommended levels of activity, and to explore how these patterns changed during pregnancy.

METHODS

This study, as part of the third phase of the Pregnancy, Infection, and Nutrition Study, investigated physical activity among 1482 pregnant women. A recall of the different modes, frequency, duration, and intensity of physical activity during the past week was assessed in two telephone interviews at 17-22 and 27-30 wk of gestation.

RESULTS

Most women reported some type of physical activity during both periods. Child and adult care giving, indoor household, and recreational activities constituted the largest proportion of total reported activity. The overall physical activity level decreased during pregnancy, particularly in care giving, outdoor household, and recreational activity. Women who were active during the second and the third trimesters reported higher levels of activity in all modes of activity than those who became active or inactive during pregnancy. The majority did not reach the recommended level of physical activity.

CONCLUSION

These data suggest that self-reported physical activity decreased from the second to the third trimesters, and only a small proportion reached the recommended level of activity during pregnancy. Further research is needed to explore if physical activity rebounds during the postpartum period.

Impact of physical activity during pregnancy and postpartum on chronic disease risk

Research over the past 20 years has focused on the safety of physical activity during pregnancy. Guidelines for health care providers and pregnant/postpartum women have been developed from the results of these studies. The overwhelming results of most studies have shown few negative effects on the pregnancy of a healthy gravida, but rather, be beneficial to the maternal-fetal unit. Recently, researchers have begun to consider the role of maternal physical activity in a more traditional chronic disease prevention model, for both mother and offspring. To address the key issues related to the role of physical activity during pregnancy and postpartum on chronic disease risk, the American College of Sports Medicine convened a Scientific Roundtable at Michigan State University in East Lansing, MI. Topics included preeclampsia, gestational diabetes, breastfeeding and weight loss, musculoskeletal disorders, mental health, and offspring health and development.

Menstrual cycle phase and oral contraceptive effects on triglyceride mobilization during exercise

We examined the effects of menstrual cycle phase and oral contraceptive (OC) use on triglyceride mobilization during 90 min of rest and 60 min of leg ergometry exercise at 45 and 65% peak O(2) uptake (Vo(2 peak)) in eight moderately physically active, eumenorrheic women (24.8 +/- 1.2 yr). Subjects were tested during the follicular phase (FP) and the luteal phase (LP) before OC use and during the inactive phase (IP) and high-dose phase (HP) after 4 complete mo of OC use. Glycerol rate of appearance (R(a)), a measure of triglyceride mobilization, was determined in a 3-h postabsorptive state using a primed constant infusion of [1,1,2,3,3-(2)H]glycerol. Before OC use (BOC), there were no significant differences between FP and LP in any of the variables studied. Dietary composition, exercise patterns, plasma glycerol concentrations, growth hormone concentrations, and exercise respiratory exchange ratio did not change with OC use. However, 4 mo of OC use significantly (P < 0.05) increased glycerol R(a) in HP during exercise at 45% Vo(2 peak) (6.2 +/- 0.2, 6.5 +/- 0.4, and 7.7 +/- 1.1 micromol.kg(-1).min(-1) for BOC, IP, and HP, respectively) and in IP and HP at 65% Vo(2 peak) (6.6 +/- 0.1, 8.2 +/- 0.6, and 8.1 +/- 0.7 micromol.kg(-1).min(-1) for BOC, IP, and HP, respectively). Plasma cortisol concentrations were significantly higher with OC use at rest and during exercise at 45 and 65% Vo(2 peak). In summary, although fluctuations of endogenous ovarian steroids have little effect on triglyceride mobilization, the synthetic ovarian steroids found in OCs increase triglyceride mobilization and plasma cortisol concentrations in exercising women. We conclude that the hierarchy of effects of ovarian steroids and their analogs on triglyceride mobilization in exercising women is as follows: energy flux > OC use > recent carbohydrate nutrition, menstrual cycle effects.

A longitudinal study of maternal oxygen saturation during short-term submaximal exercise

PURPOSE

Longitudinal prospective study before, during and after normal pregnancy of the effect of short-term submaximal exercise on maternal oxygen saturation.

METHODS

Fourteen healthy women were recruited to the study before a planned pregnancy, and were followed seven times during the pregnancy and for up to 6 month after delivery. A submaximal bicycle exercise test with a target heart rate of 85% of the predicted age-adjusted maximum was performed. Maternal oxygen saturation was continuously recorded using a pulse oximeter.

RESULTS

Maternal oxygen saturation at maximum work-load had increased significantly already at 8 weeks gestation compared with preconception levels, and remained at a significantly higher level until 29 weeks of gestation. Thereafter, the oxygen saturation continued to be higher even at 6 month postpartum. During the exercise test, the lowest saturation was found during the late recovery period, this remained unchanged before, during and after pregnancy.

CONCLUSION

A pregnant woman responds to short-term exercise by increasing the oxygen saturation until 29 weeks. After that, the saturation level decreases but remains at a higher level even 6 month after delivery compared with preconception levels.

Effects of oral contraceptives on peak exercise capacity

We examined the effects of menstrual cycle phase and oral contraceptive (OC) use on peak oxygen consumption (VO(2 peak)). Six moderately active, eumenorrheic women (25.5 +/- 1.5 yr) were studied before and after 4 mo of OC. Subjects were tested during the follicular and luteal phases before OC and the inactive and high-dose phases after OC. Before OC, there were no significant differences between the follicular and luteal phases in any of the variables studied. There were also no differences between the inactive and high-dose phases. Dietary composition, exercise patterns, and peak heart rate, minute ventilation, and respiratory exchange ratio did not change with OC use. However, OC use significantly (P </= 0.05) increased body weight (59.6 +/- 2.3 to 61.2 +/- 2.6 kg) and fat mass (13.3 +/- 1.3 to 14.5 +/- 1.3 kg) and decreased VO(2 peak) (-11%, 2.53 +/- 0.21 to 2.25 +/- 0.18 l/min). In conclusion, 1) endogenous ovarian steroids have little effect on VO(2 peak), but 2) the exogenous ovarian steroids in OC decrease peak exercise capacity in moderately physically active young women.

Exercise in pregnancy: physiological basis of exercise prescription for the pregnant woman

A pregnant woman participated in cycling events in the 2000 Olympics. Recently there was concern about the participation of a pregnant woman in the Australian netball team. More and more women are anxious to pursue sports during their pregnancies and to maintain condition. For the clinician or sports physician caring for women who want to maintain a high-level of physical activity there is no simple exercise prescription. It is probable that continuing exercise by women who are already conditioned will not result in foetal compromise, unless there are hidden or unknown complications of pregnancy. Pregnant women should probably exercise within limits that do not cause severe discomfort and should, as pregnancy progresses, be prepared to moderate the intensity and duration of their exercise programs to avoid risks and injury. It is probably not advisable for women to begin high intensity exercise programs when pregnant, although moderate exercise is beneficial to both mother and baby. The type of activity that is undertaken has to be taken into consideration and in particular the adverse effects of supine activity in late gestation recognised.

The benefits of physical activity during pregnancy

The aims of this paper are (1) to comment on the evidence relating to the health risks and benefits of physical activity (PA) for pregnant women and their unborn foetuses. and (2) to discuss the public health benefits of participation in appropriate physical activity during pregnancy. Evidence from recent original research and review papers suggests that there are potential benefits of appropriate PA in terms of maternal weight control and fitness, which are likely to have significant long term public health benefits. Concerns about the potential ill-effects of PA during pregnancy, such as hyperthermia, shortened gestational age and decreased birth weight are not supported by the most recent scientific reviews. The physiological adaptations to exercise during pregnancy appear to protect the foetus from potential harm and, while an upper level of safe activity has not been established, the benefits of continuing to be active during pregnancy appear to outweigh any potential risks. All decisions about participation in physical activity during pregnancy should however be made by women in consultation with their medical advisers.

Effects on fetal and maternal body temperatures of exposure of pregnant ewes to heat, cold, and exercise

We exposed Dorper-cross ewes at approximately 120-135 days of gestation to a hot (40 degrees C, 60% relative humidity) and a cold (4 degrees C, 90% relative humidity) environment and to treadmill exercise (2.1 km/h, 5 degrees gradient) and measured fetal lamb and ewe body temperatures using previously implanted abdominal radiotelemeters. When ewes were exposed to 2 h of heat or 30 min of exercise, body temperature rose less in the fetus than in the mother, such that the difference between fetal and maternal body temperature, on average 0.6 degrees C before the thermal stress, fell significantly by 0.54 +/- 0.06 degrees C (SE, n = 8) during heat exposure and by 0.21 +/- 0.08 degrees C (n = 7) during exercise. During 6 h of maternal exposure to cold, temperature fell significantly less in the fetus than in the ewe, and the difference between fetal and maternal body temperature rose to 1.16 +/- 0.26 degrees C (n = 9). Thermoregulatory strategies used by the pregnant ewe for thermoregulation during heat or cold exposure appear to protect the fetus from changes in its thermal environment.

Maternal physical activity in pregnancy and infant size for gestational age

PURPOSE

To estimate the effect of several types of maternal physical activity in pregnancy on size for gestational age and length of gestation.

METHODS

Telephone interviews, birth certificates, and medical records provided data on physical activity and other factors for a random sample of 291 Colorado residents. Backward polychotomous logistic regression modeling yielded estimates of the odds ratios for size for gestational age (appropriate versus small or large) and length of gestation (term versus pre-term or post-term) in relation to second and third trimester maternal physical activity.

RESULTS

Performance of any moderate or vigorous physical activity for two hours per week or more in any month was associated with a decreased risk of large infant size for gestational age (LGA; odds ratio = 0.3, 95% confidence interval = 0.2, 0.7), but had no significant effect on risk of small infant size for gestational age (SGA; odds ratio (OR) = 0.8, 95% confidence interval (CI) = 0.3, 2.3). Length of gestation was not affected by prenatal physical activity.

CONCLUSIONS

These results suggest that prenatal physical activity may decrease risk of LGA, as might be expected given its salutary effects on glucose tolerance.

Maternal leisure-time exercise and timely delivery

OBJECTIVES

This study investigated whether, in a general obstetric population, exercise in pregnancy affects the timeliness of delivery. The hypothesis was that maternal exercise would not raise the risk of preterm birth.

METHODS

A community cohort of 557 prenatal patients was followed up until the time of delivery. Data were collected on exercise in each trimester: none, low-moderate (< 1000 kcal [4184 kJ]/wk in energy expenditure), or heavy (> or = 1000 kcal/wk). Timely delivery was adopted as an outcome criterion. Thus, in the analysis, a term birth was treated as optimal and survival techniques were used to estimate risks for both preterm and postdates delivery.

RESULTS

No association was found between low-moderate exercise and gestational length. Heavier exercise appeared to reduce, rather than raise, the risk of preterm birth. The adjusted relative risk among conditioned heavy exercisers was 0.11 (95% confidence interval = 0.02, 0.81). After term, conditioned heavy exercisers delivered faster than nonexercisers.

CONCLUSIONS

The most important finding was the lack of evidence that vigorous maternal exercise is a risk factor for preterm delivery. A promising finding was that conditioned heavy exercisers have timely deliveries.

Medical limitations to wilderness travel

Medical conditions may be exacerbated by environmental stresses or may limit tolerance to them. To advise patients who are planning foreign or wilderness travel of potential health risks, medical providers must first understand the interactions between physiologic demands from environmental factors and from exercise, and the patient's medical problems. This article considers medical limitations owing to specific environmental stresses and limitations imposed by specific medical problems. The challenge is to match the prospective participant with the proposed activity to minimize the risk of failure caused by medical problems.

Maternal and fetal cardiovascular response to exercise during pregnancy

With the two-dimensional Doppler echocardiogram and M-mode echocardiogram, one can study maternal and fetal cardiovascular physiology during rest and exercise. Using such noninvasive techniques, studies indicate that left ventricular function is maintained even during vigorous bicycle exercise in healthy pregnant subjects during the second half of pregnancy. In early pregnancy, the left ventricle adapts to strenuous bicycle exercise by increasing its contractile reserve, enhancing ventricular emptying, whereas in late pregnancy, the left ventricle increases its preload reserve without significantly increasing its contractile reserve. Thus, women are "cardiovascularly" disadvantaged early in pregnancy. Using Doppler signals, early (E-passive) flow and late peak (A-active) flow reflect left ventricular diastolic filling properties. Using such techniques, we found that diastolic filling patterns are significantly influenced by pregnancy and that each trimester influences these diastolic filling patterns during upright bicycle exercise. Doppler studies of uteroplacental circulation during or after exercise have yielded conflicting results. Some have described an increase in "the vascular resistance" of this pelvic bed during strenuous exercise, whereas others have not. It seems safe to conclude that more studies are needed to elucidate this problem. Exercise does not seem to influence the resistivity index of the umbilical artery in either singleton or twins, and may even cause it to decrease. Ventricular diastolic filling properties of the fetal heart do not seem to be influenced by maternal bicycle exercise. Further studies are needed to determine if less active pregnant subjects, women with chronic hypertensive disorders, women with sickle cell anemia, or women with insulin-dependent diabetes adapt to exercise as well as their "normal" counterparts.

Physical activity and the risk of preterm labor: an epidemiological review and synthesis of recent literature

Considerable recent research effort of physical activity and pregnancy has included evaluation of preterm labor, preterm delivery, and gestational age at delivery. This review specifically evaluates recent studies that included such assessments of gestational parameters. Methodological difficulties (eg, small sample size, selective clinical populations) have prevented clear generalizable findings regarding the impact of physical activity on preterm labor and delivery. The outcomes evaluated are often times not similar from study to study and, with the associated differences in research design, prevent valid meta-analyses of this topic. Because there is some evidence that gestation and birthweight may decrease among physically active women, larger-scale, randomized clinical trials designed specifically to evaluate whether this association is simply a minor change in gestation and birthweight, or if this difference is manifested clinically in higher rates of preterm labor and delivery and low birthweight, are warranted. Without larger, population-based randomized studies, clinicians will remain unclear about the potential risks and/or benefits related to gestational duration of maternal exercise in their populations.

The use of animal models in exercise and pregnancy research

This article will discuss the use of animal models in exercise and pregnancy research. Although the ultimate goal is understanding mechanisms of change as a result of maternal exercise as they apply to pregnant women and the fetus, animal models deserve a place in research because of ethical constraints placed on experimentation on pregnant women. The risk identified through animal research should not be ignored if safe exercise guidelines are to be recommended to women who wish to engage in exercise during pregnancy.

Water exercise in pregnancy

Exercise in the water offers several physiological advantages to the pregnant woman. The hydrostatic force of water pushes extravascular fluid into the vascular spaces, producing an increase in central blood volume that may lead to increased uterine blood flow. This force is proportional to the depth of immersion. The increase in blood volume is proportional to the woman's edema. A marked diuresis and natriuresis accompanies the fluid shifts. The buoyancy of water supports the pregnant women. Water is thermoregulating. Studies of pregnant women exercising in the water have shown less fetal heart rate changes in the water than on land in response to exertion. Pregnant women's heart rates and blood pressures during water exercise are lower than on land exercise, reflecting the immersion-induced increase in circulating blood volume. The physiology of water exercise offers some compensation for the physiological changes of exercise on land that may beneficially affect pregnancy.

Applications of methods and techniques in the study of aerobic fitness during pregnancy

This article is a critical review of the general principles of exercise physiology and their relevance in pregnancy. An overview of the published studies and their conclusions are provided.