Aerobic exercise in pregnancy: an update

Physical exercise in pregnancy: benefits, risks and prescription

OBJECTIVES

The aim of this article is to provide a comprehensive literature review, gathering the strongest evidence about the risks and benefits and the prescription of physical exercise during pregnancy.

CONTENT

Regular physical exercise during pregnancy is associated with numerous benefits. In general women are not adequately advised on this matter. Along with their concerns regarding the potential associated risks, it contributes to the abandonment or refusal to start exercising during pregnancy. A systematic review was conducted in MEDLINE including articles considered to have the highest level of scientific evidence. Fifty-seven articles, including 32 meta-analysis, 9 systematic reviews and 16 randomized controlled trials were included in the final literature review.

SUMMARY

Exercise can help preventing relevant pregnancy related disorders, such as gestational diabetes, excessive gestational weight gain, hypertensive disorders, urinary incontinence, fetal macrosomia, lumbopelvic pain, anxiety and prenatal depression. Exercise is not related with an increased risk of maternal or perinatal adverse outcomes. Compliance with current guidelines is sufficient to achieve the main benefits, and exercise type and intensity should be based on woman's previous fitness level.

OUTLOOK

Exercise in pregnancy is safe for both mother and fetus, contributing to prevent pregnancy related disorders. Exercise type and intensity should be adapted to woman's previous fitness level, medical history and characteristics of the ongoing pregnancy.

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.

Estimated energy requirements increase across pregnancy in healthy women with dichorionic twins

Background

Estimated energy requirement (EER) has not been defined for twin pregnancy. This study was designed to determine the EER of healthy women with dichorionic-diamniotic (DCDA) twin pregnancies.

Objectives

We aimed to estimate energy deposition from changes in maternal body protein and fat; to measure resting energy expenditure (REE), physical activity level (PAL), and total energy expenditure (TEE) throughout pregnancy and postpartum; and to define the EER based on the sum of TEE and energy deposition for twin gestation.

Design

This is a prospective study of 20 women with DCDA twin gestations. Maternal EER, energy deposition, REE, TEE, and PAL were obtained during the first, second, and third trimesters of pregnancy and immediately postpartum. A mixed-effects linear regression model for repeated measures with random intercept was used to test for the effects of BMI groups and time.

Results

Gains in total body protein (mean ± SD: 2.1 ± 0.7 kg) and fat mass (5.9 ± 2.8 kg) resulted in total energy deposition of 67,042 ± 25,586 kcal between 0 and 30-32 weeks of gestation. REE increased 26% from 1392 ± 162 to 1752 ± 172 kcal/d across the 3 trimesters, whereas TEE increased 17% from 2141 ± 283 to 2515 ± 337 kcal/d. Physical activity decreased steadily throughout pregnancy. Reductions in physical activity did not compensate for the rise in REE and energy deposition, thus requiring an increase in dietary energy intake as pregnancy progressed. EER increased 29% from 2257 ± 325 kcal/d in the first trimester to 2941 ± 407 kcal/d in the second trimester, and stayed consistent at 2906 ± 350 kcal/d in the third trimester.

Conclusion

Increased energy intake, on average ∼700 kcal/d in the second and third trimesters when compared with the first trimester, is required to support gestational weight gain and the rise in energy expenditure of DCDA twin pregnancies.

Evidence-based risk assessment and recommendations for physical activity clearance: Consensus Document 2011

The Physical Activity Readiness Questionnaire (PAR-Q) and the Physical Activity Readiness Medical Evaluation (PARmed-X) are internationally known preparticipation screening tools developed on the basis of expert opinion. The primary purposes of this consensus document were to seek evidence-based support for the PAR-Q and PARmed-X forms, to identify whether further revisions of these instruments are warranted, to determine how people responding positively to questions on the PAR-Q can be safely cleared without medical referral, and to develop exercise clearance procedures appropriate for various clinical conditions across the human lifespan. Seven systematic reviews were conducted, examining physical-activity-related risks and effective risk-stratification procedures for various prevalent chronic conditions. An additional systematic review assessed the risks associated with exercise testing and training of the general population. Two gap areas were identified and evaluated systematically: the role of the qualified exercise professional and the requisite core competencies required by those working with various chronic conditions; and the risks associated with physical activity during pregnancy. The risks associated with being physically inactive are markedly higher than transient risks during and following an acute bout of exercise in both asymptomatic and symptomatic populations across the lifespan. Further refinements of the PAR-Q and the PARmed-X (including online versions of the forms) are required to address the unique limitations imposed by various chronic health conditions, and to allow the inclusion of individuals across their entire lifespan. A probing decision-tree process is proposed to assist in risk stratification and to reduce barriers to physical activity. Qualified exercise professionals will play an essential role in this revised physical activity clearance process.

Travel to high altitude during pregnancy: frequently asked questions and recommendations for clinicians

The effects of altitude on pregnancy have been extensively studied in high altitude residents, but there is a lack of knowledge concerning the pregnant altitude visitor. Exposure to hypoxia results in physiologic responses which act to preserve maternal and fetal oxygenation. However, these reactions are limited and maternal/fetal complications may be observed, especially in association with exercise. Certain pre-existing conditions or risk factors of hypertension/preeclampsia and/or fetal growth restriction are contra-indications for traveling to high altitude, especially after 20 weeks. The acclimatization process has to be respected to avoid acute mountain sickness without taking drugs, and at least a few days of acclimatization are required before exercising.

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.

Maternal exercise reduces hyperthermia-induced apoptosis in developing mouse brain

PURPOSE

Hyperthermia-induced apoptosis is mediated by mitochondrial pathway, and is temporally correlated with alterations in mitochondrial morphology in neuroepithelial cells. In addition, regular exercise up-regulates heat shock proteins (HSPs) that inhibit apoptosis. However, embryo-protective effects of maternal exercise against heat exposure during pregnancy have not been fully understood yet.

MATERIALS AND METHODS

To investigate the role of maternal exercise in protecting embryos from hyperthermia, we measured apoptosis-related factors and HSPs in Hsp70 knockout mouse embryos. Pregnant mice were divided into control, exercise, hyperthermia-after-exercise, and hyperthermia groups. Where appropriate the swimming exercise was performed for 5-10 min/day from embryonic day (ED) 1 to ED 8, and hyperthermia (43°C, 5 min) was induced on ED 8. To characterise the effects of maternal exercise on apoptosis-related factors and HSPs, we performed western blotting and transmission electron microscopy.

RESULTS

Caspase-9, -7, -3 and Bax were down-regulated in the hyperthermia-after-exercise group and Bcl-2, Hsp27 and Hsp110 were up-regulated. The number of apoptotic cells was markedly reduced in the hyperthermia-after-exercise group.

CONCLUSIONS

Maternal exercise plays an important role in inhibiting apoptotic cell death in embryos against hyperthermic exposure during pregnancy.

Evidence-based risk assessment and recommendations for physical activity clearance: pregnancy1This paper is one of a selection of papers published in this Special Issue, entitled Evidence-based risk assessment and recommendations for physical activity clearance, and has undergone the Journal’s usual peer review process

Recommendations for physical activity during pregnancy have progressed significantly in the last 30 years and continue to evolve as an increasing body of scientific evidence becomes available. In addition, there is an increasing number of women who wish to either maintain physical fitness levels during the prenatal period or initiate exercise for a healthier lifestyle during pregnancy. As such, consistent evaluation of the risks associated with exercise during pregnancy is warranted for maternal and fetal well-being. The primary purpose of this systematic review was to evaluate the scientific information available regarding maternal and fetal responses as it relates to the occurrence of adverse exercise-related events in pregnant women without contraindications to exercise. A systematic and evidence-based approach was used to analyze critically the level of evidence for risks associated with acute and chronic exercise during pregnancy in healthy pregnant women. All articles were screened according to standardized evaluation criteria developed by a panel of experts. A total of 74 investigations that involved 3766 pregnant women were included in the analysis. Of the 74 studies, only 35 studies documented the presence or absence of adverse events. The serious adverse event rate for these studies was 1.4 per 10 000 h of exercise. The adverse event rate increased to 6.8 per 10 000 h of exercise when including the occurrence of more mild events and exercise-related fetal bradycardia and tachycardia. Previously inactive or active women (without contraindications) are at a low risk for adverse fetal or maternal events if they participate in routine physical activity during pregnancy.

Development and validation of exercise target heart rate zones for overweight and obese pregnant women

Validated target heart rate (THR) zones for exercise prescription for overweight and obese pregnant women have not been developed. The purposes of this study were to determine if heart rate reserve (HRreserve) is best described by aerobic capacity at peak exercise or by aerobic capacity reserve (VO2 reserve) and to develop and validate THR zones for light-intensity exercise (20%–39%VO2 reserve) in sedentary overweight and obese pregnant women. One hundred six women between 16 and 20 weeks gestation with medical clearance performed a progressive treadmill test to volitional fatigue (peak). Data from every 4th subject were used for cross-validation. Two linear regression equations were performed for each subject, then pooled to obtain mean group values (± SD): %HRreserve vs. %VO2 peak and %HRreserve vs. %VO2 reserve. THR zones equivalent to 20%–39%VO2 reserve were developed and validated based on the strongest relationship. %HRreserve had a stronger linear relationship with %VO2 reserve (y = 1.046x –7.561; R2 = 0.741) than %VO2 peak (y = 1.259x –28.795; R2 = 0.604). Validated THR ranges for sedentary overweight and obese pregnant women are 102–124 beats·min–1 (20–29 years of age) and 101–120 beats·min–1 (30–39 years of age), representing an exercise intensity of 20%–39%VO2 reserve as recommended by the American College of Sports Medicine for previously sedentary pregnant women. Overweight and obese women who are medically prescreened can exercise during pregnancy within our validated THR zones. The relationship between HR and VO2 remains strong, but the two are not equivalent in this population group.

Chemical and mechanical adaptations of the respiratory system at rest and during exercise in human pregnancy

Human pregnancy is characterized by significant increases in ventilatory drive both at rest and during exercise. The increased ventilation and attendant hypocapnia of pregnancy has been attributed primarily to the stimulatory effects of female sex hormones (progesterone and estrogen) on central and peripheral chemoreflex drives to breathe. However, recent research from our laboratory suggests that hormone-mediated increases in neural (or non-chemoreflex) drives to breathe may contribute importantly to the hyperventilation of pregnancy. This review challenges traditional views of ventilatory control, and outlines an alternative hypothesis of the control of breathing during human pregnancy that is currently being tested in our laboratory. Conventional wisdom suggests that pregnancy-induced increases in central respiratory motor output command in combination with progressive thoraco–abdominal distortion may compromise the normal mechanical response of the respiratory system to exercise, increase the perception of exertional breathlessness, and curtail aerobic exercise performance in otherwise healthy pregnant women. The majority of available evidence suggests, however, that neither pregnancy nor advancing gestation are associated with reduced aerobic working capacity or increased breathlessness at any given work rate or ventilation during exhaustive weight-supported exercise.

V̇O2peak Prediction and Exercise Prescription for Pregnant Women

PURPOSE

The present study was designed to develop and validate a prediction equation for peak oxygen consumption VO2peak) using a progressive treadmill test and to refine the current target HR exercise guidelines for pregnancy (PARmed-X for Pregnancy).

METHODS

One hundred fifty-six women between 16 and 22 wk of gestation performed the test to volitional fatigue (peak exercise test). Data from every fourth subject were used to form the cross-validation group. The women were separated into two age groups; 20-29 (N = 60) and 30-39 (N = 96) yr of age and then further separated into fit (VO2peak at the top 25th percentile), unfit (VO2peak at the bottom 25th percentile), and active (between these two ranges). HR and VO2peak values were used in the regression equation to predict target HR ranges at 60 and 80% VO2peak.

RESULTS

The prediction equation (R2 = 0.72, R2adjusted = 0.71 and SEE = 2.7) was compared with cross validation (N = 39; P = 0.78). Fit women had a VO2peak > or = 27.2 mL.kg(-1).min(-1) and > or = 26.1 mL.kg.min for ages 20-29 and 30-39 yr, respectively, representing the top 25th percentile. Unfit women had a VO2peak of < or = 21.0 mL.kg(-1).min(-1) and < or = 19.6 mL.kg(-1).min(-1), respectively, representing the bottom 25th percentile. HR/VO2peak regression lines for each fitness level were used to generate the target HR zones in each age group.

CONCLUSION

This is the first study to provide a validated prediction equation of VO2peak for pregnant women using a progressive treadmill exercise test. The defined target HR zones based on age and the appropriate fitness levels can be used for exercise prescription in healthy pregnant women.

Energy requirements during pregnancy based on total energy expenditure and energy deposition

BACKGROUND

Energy requirements during pregnancy remain controversial because of uncertainties regarding maternal fat deposition and reductions in physical activity.

OBJECTIVE

This study was designed to estimate the energy requirements of healthy underweight, normal-weight, and overweight pregnant women and to explore energetic adaptations to pregnancy.

DESIGN

The energy requirements of 63 women [17 with a low body mass index (BMI; in kg/m(2)), 34 with a normal BMI, and 12 with a high BMI] were estimated at 0, 9, 22, and 36 wk of pregnancy and at 27 wk postpartum. Basal metabolic rate (BMR) was measured by calorimetry, total energy expenditure (TEE) by doubly labeled water, and activity energy expenditure (AEE) as TEE - BMR. Energy deposition was calculated from changes in body protein and fat. Energy requirements equaled the sum of TEE and energy deposition.

RESULTS

BMR increased gradually throughout pregnancy at a mean (+/-SD) rate of 10.7 +/- 5.4 kcal/gestational week, whereas TEE increased by 5.2 +/- 12.8 kcal/gestational week, which indicated a slight decrease in AEE. Energy costs of pregnancy depended on BMI group. Although total protein deposition did not differ significantly by BMI group (mean for the 3 groups: 611 g protein), FM deposition did (5.3, 4.6, and 8.4 kg FM in the low-, normal-, and high-BMI groups; P = 0.02). Thus, energy costs differed significantly by BMI group (P = 0.02). In the normal-BMI group, energy requirements increased negligibly in the first trimester, by 350 kcal/d in the second trimester, and by 500 kcal/d in the third trimester.

CONCLUSION

Extra energy intake is required by healthy pregnant women to support adequate gestational weight gain and increases in BMR, which are not totally offset by reductions in AEE.

Aerobic conditioning effects on substrate responses during graded cycling in pregnancy

This study was conducted to test the hypothesis that aerobic conditioning prevents exercise-induced hypoglycemia and preserves the capacity to utilize carbohydrates and to produce lactate during heavy exercise in late gestation. The effects of closely monitored cycle ergometer conditioning (heart rate = 143 +/- 2 beats/min, 25 min/day, 3 days/week) during the second and third trimesters were studied in 18 previously sedentary women (exercised group, EG). A nonexercising pregnant control group (CG, n = 9) was also studied. Data collection times for both groups were as follows: start of the second trimester (Entry), ends of the second (TM2) and third (TM3) trimesters (post-training), and 4-6 months postpartum (nonpregnant control). Respiratory gas exchange was studied and venous blood samples were obtained before, during, and after a graded cycle ergometer test that was terminated at a peak heart rate of 170 beats/min. Measurements included plasma glucose, insulin, free fatty acids, the respiratory exchange ratio at peak exercise, and peak postexercise lactate concentration. A significant aerobic conditioning effect in the EG was confirmed by a 17% increase in O2 pulse at peak exercise between Entry and TM3. As expected, values for free fatty acids in the CG rose with advancing gestational age. The CG showed a clear trend for a rise in plasma insulin with advancing gestational age, under all experimental conditions. Also, peak exercise respiratory exchange ratio and peak postexercise lactate concentration were significantly reduced in late gestation, and plasma glucose decreased significantly during and following the end of TM3 testing. Effects of pregnancy to reduce peak postexercise lactate and to reduce plasma glucose during and after exercise at the end of the third trimester were significantly attenuated in the EG. These effects were attributed to attenuation of pregnancy-induced insulin resistance (as reflected by insulin/glucose ratio) by physical conditioning. These findings support our original experimental hypothesis that aerobic conditioning prevents exercise-induced hypoglycemia and preserves the ability to utilize carbohydrate and produce lactate during heavy exercise in late gestation.

Clinical physiology of exercise in pregnancy: a literature review

OBJECTIVES

To review the existing literature on the physiology of exercise in pregnancy as a basis for clinical practice guidelines for prenatal exercise prescription.

METHODS

MEDLINE search for English language abstracts and articles published between 1966 and 2003 related to physiological adaptations to pregnancy, effects of pregnancy on responses to acute exercise and aerobic conditioning, effects of acute maternal exercise on indexes of fetal well-being, impact of physical conditioning on birth weight and other pregnancy outcomes, and use of exercise to prevent or treat gestational diabetes mellitus and preeclampsia.

RESULTS

Maximal aerobic power (VO(2)max, L/min) is well-preserved in pregnant women who remain physically active, but anaerobic working capacity may be reduced in late gestation. The increase in resting heart rate, reduction in maximal heart rate, and resulting smaller heart rate reserve render heart rate a less precise way of estimating exercise intensity. As rating of perceived exertion (RPE) is not altered by pregnancy, the use of revised pulse rate target zones along with Borg's RPE scale is recommended to prescribe exercise intensity during pregnancy. Responses to prolonged submaximal exercise (>30 min) in late gestation include a moderate reduction in maternal blood glucose concentration, which may transiently reduce fetal glucose availability. The normal response to sustained submaximal exercise is an increase in fetal heart rate (FHR) baseline. Transient reductions in FHR reactivity, fetal breathing movements, and FHR variability may also occur in association with more strenuous exercise. Controlled prospective studies have demonstrated that moderate prenatal exercise during the second and third trimesters is useful to improve aerobic fitness and maternal-fetal physiological reserve without affecting fetal growth.

CONCLUSIONS

The Physical Activity Readiness Medical Examination for Pregnancy is recommended for use by physicians and midwives to provide medical clearance for prenatal exercise. Evidence-based prenatal exercise guidelines are needed to counsel healthy and fit pregnant women on the safety of involvement in more strenuous physical conditioning. Future study is also recommended to determine the usefulness of prenatal exercise in the prevention and treatment of gestational diabetes mellitus and preeclampsia.

Plasma acid-base regulation above and below ventilatory threshold in late gestation

Stewart's physicochemical approach was used to study the effects of pregnancy on acid-base regulation in arterialized blood. Responses of 15 healthy pregnant women (PG; gestational age, 37.1 +/- 0.2 wk) were compared with those of 15 nonpregnant controls (CG) at rest and during cycling at 70 and 110% of the ventilatory threshold (T(vent)). Hydrogen ion concentration ([H(+)]) was lower in the PG vs. CG at rest and during exercise (P < 0.05 at rest and 70% T(vent)). Exercise-induced changes in [H(+)] were similar between groups. Lower resting [H(+)] values in the PG vs. CG resulted from lower values for arterialized PCO(2) (Pa(CO(2))) and total weak acid ([A](tot)), which were partly offset by a lower strong-ion difference ([SID]). Reductions in [A](tot) and [SID] at rest were primarily the result of reductions in albumin [Alb] and sodium [Na(+)], respectively. In the transition from rest to 70% T(vent), small increases in Pa(CO(2)) and [A](tot) contributed to moderate increases in [H(+)] in both groups, however [SID] increased in the PG and decreased in the CG (P < 0.05 between groups). In the transition from rest to 110% T(vent), decreases in [SID] made a significantly greater contribution to changes in [H(+)] in the CG vs. PG. Exercise-induced increases in [H(+)] are similar in the pregnant vs. nonpregnant state, but there is a reduced contribution of [SID] both above and below T(vent) during pregnancy.

Physical conditioning effects on fetal heart rate responses to graded maternal exercise

PURPOSE

This study examined the effects of advancing gestational age and maternal aerobic conditioning (stationary cycling) on fetal heart rate (FHR) responses to strenuous non-steady-state maternal exercise.

METHODS

Subjects chose to participate in either an exercise group (EG) or control group (CG). Fourteen healthy, previously sedentary pregnant women participated in the exercise group, and six pregnant controls remained sedentary. Stationary cycling (heart rate target: 145 beats x min(-1)) was performed 3 d x wk(-1) by the exercised group. Exercise duration was increased from 14 to 25 min x session(-1) during the second trimester and was maintained at 25 min x session(-1) throughout the third trimester. FHR was monitored before, during, and after a progressive submaximal cycle ergometer test (peak heart rate = 170 beats x min(-1)) performed at approximately 27 and 37 wk gestation.

RESULTS

Mean FHR increased significantly (P < 0.05) during exercise, followed by a modest suppression and then a delayed rise during the recovery period at both observation times. Fetal bradycardia was not observed in any of the exercise tests. Effects of advancing gestational age included a lower FHR baseline both at rest and in response to maternal exercise and a lower incidence of exercise-induced tachycardia. Maternal physical conditioning did not significantly alter FHR response to maternal exercise.

CONCLUSION

Our results support the hypothesis that FHR responses to strenuous exercise are altered by advancing gestational age and a brief progressive exercise test terminated at a maternal heart rate of 170 beats x min(-1) does not induce fetal distress during a healthy pregnancy.

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.

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.

Substrate and hormonal responses during exercise classes at selected stages of pregnancy

This study examined the substrate and hormonal responses during exercise classes for 36 pregnant women at selected stages of pregnancy. Six nonpregnant women served as controls. Blood samples were obtained before exercise, at the end of aerobic exercise, and 45 min after aerobic exercise. All samples were analyzed for substrates (glucose, lactate, FFA) and 7 hormones. Heart rates in the pregnant women were approximately 150 bpm, although some were as high as 180 bpm. Heart rates in the control group were approximately 130 bpm. Modest increments occurred in lactate and FFA during exercise, and decrements in glucose were observed in all groups. Insulin concentrations decreased with exercise and remained depressed after exercise whereas FFA remained elevated postexercise in the pregnant women. Modest transient changes were observed with exercise in the other hormones. A severely blunted norepinephrine response to exercise was observed in the third-trimester group. In conclusion, uncontrolled non-steady-state exercise does not disrupt the substrate and endocrine milieu markedly. Recovery in hormonal and substrate concentrations appears to occur rapidly after exercise. However, decrements in blood glucose do occur for a short time during prenatal exercise classes, most notably for women in the third trimester.