REVIEW ARTICLESports Med 2010; 40 (6): 493-607

0112-1642/10/(XX)6-0493/S49 96/0

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Physical Activity and PregnancyCardiovascular Adaptations, Recommendations and PregnancyOutcomes

Katarina Melzer}'^ Yves Schutz^ Michel Boulvain^ and Bengt Kayser^

1 Institute of Movement Sciences and Sports Medicine, Faculty of Medicine, University of Geneva, Geneva,Switzerland

2 Department of Obstetrics and Gynaecology, University Hospitals of Geneva, Faculty of Medicine,University of Geneva, Geneva, Switzerland

3 Department of Physiology, University of Lausanne, Lausanne, Switzerland

ContentsAbstract 4931, Cardiovascular Adaptations to Training and Detraining in Pregnant and Nonpregnant States , , , , 495

1.1 Cardiovascular Adaptations to Training 495.,2 Cardiovascuiar Adaptations to Detraining 495

2, Cardiovascuiar Changes due to Pregnancy 4962.1 Cardiovascular Ci^anges during Labour and Deiivery 4962.2 Postpartum Cardiovascular Cinanges 497

3, Piiysicai Activity and Pregnancy 4973.1 Submaximai Aerobic Capacity during Pregnancy 4973.2 Wori< Efficiency 4983.3 Maximum Aerobic Capacity during Pregnancy 4983.4 Maximai Heart Rate during Pregnancy 498

4, Pir/sical Activity Recommendations during Pregnancy 4984,1 Recommended iHeart Rate Target Zones for Aerobic Exercise in Pregnancy 500

5, Compiiance witii Pi ysicai Activity Recommendations during Pregnancy 5006, Effects of Phvsicai Activity on Pregnancy Outcomes 5017, Conclusions 503

Abstract Regular physical activity is associated with improved physiological, meta-bolic and psychological parameters, and with reduced risk of morbidity andmortality. Current recommendations aimed at improving the health and well-being of nonpregnant subjects advise that an accumulation of >30 minutes ofmoderate physical activity should occur on most, if not all, days of the week.

Regardless of the specific physiological changes induced by pregnancy,which are primarily developed to meet the increased metabolic demands ofmother and fetus, pregnant women benefit from regular physical activity thesame way as nonpregnant subjects.

Changes in submaximal oxygen uptake (VO2) during pregnancy dependon the type of exercise performed. During maternal rest or submaximalweight-bearing exercise (e.g. walking, stepping, treadmill exercise), absolute

494 Melzer et ai

maternal VO2 is significantly increased compared with the nonpregnant state.The magnitude of change is approximately proportional to maternal weightgain.

When pregnant women perform submaximal weight-supported exerciseon land (e.g. level cycling), the findings are contradictory. Some studies re-ported significantly increased absolute VO2, while many others reportedunchanged or only slightly increased absolute VOT compared with the non-pregnant state. The latter findings may be explained by the fact that themetabolic demand of cycle exercise is largely independent of the maternalbody mass, resulting in no absolute VO2 alteration.

Few studies that directly measured changes in maternal maximal VO2(V02niax) showed no difference in the absolute V02max between pregnant andnonpregnant subjects in cycling, swimming or weight-bearing exercise. Effi-ciency of work during exercise appears to be unchanged during pregnancy innon-weight-bearing exercise. During weight-bearing exercise, the work effi-ciency was shown to be improved in athletic women who continue exercisingand those who stop exercising during pregnancy. When adjusted for weightgain, the increased efficiency is maintained throughout the pregnancy, withthe improvement being greater in exercising women.

Regular physical activity has been proven to result in marked benefits formother and fetus. Maternal benefits include improved cardiovascular func-tion, limited pregnancy weight gain, decreased musculoskeletal discomfort,reduced incidence of muscle cramps and lower limb oedema, mood stability,attenuation of gestational diabetes mellitus and gestational hypertension.Fetal benefits include decreased fat mass, improved stress tolerance, andadvanced neurobehavioural maturation. In addition, few studies that havedirectly examined the effects of physical activity on labour and delivery in-dicate that, for women with normal pregnancies, physical activity is accom-panied with shorter labour and decreased incidence of operative delivery.

However, a substantial proportion of women stop exercising after theydiscover they are pregnant, and only few begin participating in exerciseactivities during pregnancy. The adoption or continuation of a sedentarylifestyle during pregnancy may contribute to the development of certain dis-orders such as hypertension, maternal and childhood obesity, gestationaldiabetes, dyspnoea, and pre-eclampsia. In view of the global epidemic ofsedentary behaviour and obesity-related pathology, prenatal physical activitywas shown to be useful for the prevention and treatment of these conditions.Further studies with larger sample sizes are required to confirm the associa-tion between physical activity and outcomes of labour and delivery.

Regular physical activity is associated with the health and well-being of nonpregnant sub-improved physiological, metabolic and psycho- jects advise that an accumulation of 30 minuteslogical parameters, and with reduced risk of or more of moderate physical activity should oc-morbidity and mortahty from diseases such as cur on most, if not all, days of the week.'^'cardiovascular disease, hypertension, diabetes Regardless of the specific physiological chan-mellitus, obesity, osteoporosis, sarcopenia, cog- ges induced by pregnancy, which are primarilynitive disorders and some forms of cancer.'"' developed to meet increased metabolic demandsCurrent recommendations aimed at improving of mother and fetus, pregnant women benefit

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Physical Activity and Pregnancy 495

from regular physical activity the same way asnonpregnant subjects.' ' However, a substantialproportion of women stop exercising and de-crease their general physical activity level afterthey discover they are pregnant, and only fewbegin participating in exercise or sport activitiesduring pregnancy.''*' The adoption or continua-tion of a sedentary lifestyle during pregnancymay contribute to development of certain dis-orders such as hypertension, maternal and child-hood obesity, gestational diabetes, dyspnoea andpre-eclampsia.'- ' In view of the global epidemic ofsedentary behaviour and obesity-related patho-logy, prenatal physical activity has been shown tobe useful for the prevention and treatment ofthese conditions.'^'

A systematic literature review was conductedon physical activity and pregnancy. The searchincluded articles published in MEDLINE and ISIWeb of Science databases. Keywords used were:'physical activity' OR 'physical exercise', 'preg-nancy' OR 'gestation', 'pregnancy outcomes','labour', 'cardiovascular adaptations', 'heart rate','training', 'detraining', 'physical activity recom-mendations' AND 'pregnancy'. In a first round,there were no restrictions to certain yearsof publication. In a second round, publicationspublished between 2007 and 2009 were specifi-cally reviewed to assure inclusion of any relevantnew publication.

The aim of this article was to review the cur-rent state of knowledge on (i) the cardiovascularadaptations to physical activity in the pregnantand nonpregnant states; (ii) the compliance ofpregnant women with current physical activityrecommendations; and (iii) the effects of physicalactivity on pregnancy outcomes.

1. Cardiovascular Adaptations to Trainingand Detraining in Pregnant andNonpregnant States

1.1 Cardiovascular Adaptations to Training

Repeated episodes of physical activity per-formed over a longer period (i.e. training) causeadaptations in the respiratory, cardiovascularand neuromuscular systems that enable physi-

cally trained persons to exercise for longer ata given absolute exercise intensity, or to ex-ercise at a higher exercise intensity for a givenduration.'^' The adaptations in metabolic andphysiological systems depend on the type of ex-ercise overload imposed. Short duration activitiesdemanding high levels of anaerobic metabohsmfavour the adaptation of the immediate andshort-term energy systems, with limited impacton the aerobic system. Regular endurance train-ing, on the other hand, improves overall aerobiccapacity.' "' 1

For public health concerns and in contrast totraining for sports-specific improvement, currentinterest in physical activity participation ariseslargely from a desire to improve health-relatedfitness components, primarily cardiorespiratoryfitness.''"' Aerobic training produces significantchanges to the cardiovascular system: enlargedleft ventricular cavity of the heart; enhancedblood and stroke volume; increased maximumcardiac output (Q); and decreased resting and sub-maximal exercise heart rate.' '' The lower restingand sub-maximum exercise heart rate generallyreflect an improved submaximal and maximaloxygen uptake (VO2max) and a correspondinglyhigher level of cardiovascular fitness.

1,2 Cardiovascular Adaptations to Detraining

While regular physical activity is accompaniedby better cardiovascular fitness, a reduction orcessation of physical activity leads to partialor complete reversal of the physiological adap-tations. Inactivity is accompanied by a rapiddecline in VO max and blood volume. Conse-quently, submaximal exercise heart rate increasesbut insufficiently to counterbalance decreasedstroke volume, thus resulting in a reduction ofmaximal Q.'"' Measurable alterations in physio-logical functions take place after only a week ortwo of detraining.'**! Total loss of training im-provements occurs within several months.'**' As aresult, any sudden physical effort imposed on thedetrained subjects leads to physiological andmetabolic stress, as they are not able to respondto imposed physical exertion as efficiently astrained subjects.

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496 Melzeret al.

2. Cardiovascular Changes due toPregnancy

Regardless of training status, women who be-come pregnant undergo profound cardiovascularsystem alterations (figure 1). The first haemody-namic change during pregnancy is a rise in heartrate, both at rest and during submaximal work-out. It starts between 2 and 5 weeks of pregnancyand continues well into the third trimester.I'^^ Onaverage, the resting heart rate raises 8 beats/minby the eighth week, and reaches an increase of16 beats/min by the end of pregnancy.''^' The ef-fect is less evident in supine or lateral positionsand more evident during sitting.'"'*' The mechan-ism of the increased heart rate is not yet clearlyidentified. It may be attributed to chorionic gona-dotropin, or to sympathetic reflex adjustments tomaintain arterial blood pressure despite reducedperipheral vascular resistance.'''''

Between 10 and 20 weeks of pregnancy, a no-table increase in blood volume takes place due to anincrease in both plasma and erythrocytes.''*'^^ Thisrepresents a rise of approximately 1500mL,''^ ofwhich 1000 mL is plasma volume and 500 mL iserythrocytes.t'**' Since plasma volume amplifiesmore than red blood cell volume, a relative dilu-tional anaemia occurs.'"' Blood volume expansionmay be even greater in multifetal gestations.''^'

Resting Q is increased as early as the fifth weekof pregnancy as a result of the increased heart

rate, stroke volume and blood volume.''^-Resting Q increases by 1 L/min at 8 weeks of gest-ation, which represents >50% of the overallchange in pregnancy.'^°' During the third trime-ster, resting Q increases only minimally, primar-ily because of the increase in heart rate as termapproaches.'-^'I In multifetal pregnancies, restingmaternal Q is greater by approximately 20%compared with singleton pregnancies.'"' Q is alsoaffected by the positional changes of thewomen.''"•' After 20 weeks of gestation, the graviduterus may obstruct the aorta and inferior venacava, causing a decrease in uteroplacental bloodflow and venous return to the heart,''' especiallywhen the woman is in the supine position. Theleft lateral position quickly relieves compressionof the inferior vena cava.'*'' Left uterine dis-placement also tends to prevent aortocaval com-pression, although it is less optimal than the leftlateral position.''**'

Blood pressure is not increased in normalpregnancy due to decreased peripheral vascularresistance.''^1 In fact, systolic pressure remainsquite stable, whereas diastoiic pressure decreasesup to 15 mmHg in '

2.1 Cardiovascular Changes during Labourand Delivery

Although Q remains relatively constant in thelatter half of pregnancy, there is a significant

Increase in resting and Increase in resting andsub-maximal heart rate

Descrease inblood pressure

Increase in plasma andred blood cell massPregnancy-induced

cardiovascular changes

ncrease in blood volume

Increase in stroke volume

Fig. 1. Pregnancy-Induced cardiovascular changes. Q = cardiac output; V02niai = 'Ti3ximal oxygen uptake.

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Physical Activity and Pregnancy 497

increase during active labour and immediately afterdelivery. A study''^' of normal labour in womenwithout epidural anaesthesia reported an increasein Q of 12% and 34% between contractions and atfull dilatation, respectively. The increase in Qseems to be caused by an increase in heart rateand stroke volume. Such changes are thought tobe sympathetically mediated and are likely due tothe combined effects of pain, increased metabolicdemand and increased venous return duringuterine contractions.'*"*' Circulating blood vol-ume also increases during contractions by anadditional 300-500 mL due to blood auto-transfusion from the placenta.'-"*'

The physical effort of normal labour doesnot impose high energy demands on the parturi-ent. The energy requirement is affected more bythe frequency and duration of uterine contrac-tions than by the total duration of labour.'^^'Katz et al. measured energy expenditure of la-bour in 23 healthy women.' - ' The results showedoxygen uptake (VO2) of 0.255 L/min during uter-ine relaxation (at 4 cm dilatation), 0.338 L/minduring contractions (at 4 cm dilatation), and0.510 L/min at delivery. Thus, the VO2, whichincreases -20% during normal pregnancy, mayincrease an additional 60% during the contrac-tions, but remains rather low when comparedwith the increase observed during physical ac-tivity (walking, running, cychng). It is higher inmultiparous women than in nulliparous womenand is highest in those women with the shortestlabour.'-^) If the process of expulsion is pro-longed (>30 minutes), the increased demand foroxygen is partially met by anaerobic metabolismcausing an increase in maternal blood lactate

'25'

The haemodynamic changes seen during la-bour and delivery are infiuenced by anaestheticand analgesic techniques.'-'' Lumbar epiduralanaesthesia during labour reduces maternaladrenaline (epinephrine) levels,'^^' and decreasesthe work of breathing, VO2,'"' **' fetal heartrate,'^''' maternal Q and blood pressure.'-'O"-^-' Thehaemodynamic changes are also influenced by theposition of the parturient. For example, Q andstroke volume are significantly decreased in thesupine, compared with the lateral, position.'-'-^'

2.2 Postpartum Cardiovascular Changes

Within the first 15-20 minutes after deliveryof the fetus and placenta there is a substantialincrease in Q, as the blood is no longer divertedto the uteroplacental vascular bed, but ratherredirected to the maternal circulation.'-'' By24 hours after delivery, Q is no longer signifi-cantly different from pre-labour values,'^'*' andfully returns to pre-pregnant values by 2 weeksafter delivery. Stroke volume also decreaseswithin 2 weeks, although there is a further smallreduction up to 6 months after delivery.''^'

3. Physical Activity and Pregnancy

Although pregnancy induces an increase in Q,stroke volume and heart rate, women who con-tinue aerobic exercise training during pregnancyhave lower resting heart rate and higher strokevolume than matched sedentary controls.'-^•''-^ ' Inaddition, aerobically fit women have greater VO2response at a given heart rate compared withtheir sedentary counterparts.'-^^'

3.1 Submaximal Aerobic Capacity duringPregnancy

Changes in submaximal VO2 during preg-nancy depend on the type of exercise performed.During maternal rest or submaximal weight-bearing exercise (e.g. walking, stepping, treadmillexercise), absolute maternal VO2 (L/min) is signi-ficantly increased compared with the nonpreg-nant state.'- - **' The magnitude of change isapproximately proportional to maternal weightgain. At the same speed or grade of walking orrunning, the values for VO2 expressed in mL/kg/minare thus similar or only slightly higher duringpregnancy compared with the nonpregnant

'"3'4"

When pregnant women perform submaximalweight-supported exercise on land (e.g. level cy-cling), where the energy cost of locomotion is notaltered by maternal morphological changes, thefindings are contradictory. Some studies reportedsignificantly increased absolute V02,''^'''°'''^'while many others''5-'' - '*''*"'*''-4 ' reported un-changed or only slightly increased absolute VO2

© 2010 Adis Data information BV. All rights reserved. Sports Med 2010; 40 (6)

498 Melzer et al.

compared with the nonpregnant state. The latterfindings may be explained by the fact that themetabolic demand of cycle exercise is largely in-dependent of the maternal body mass, resultingin no absolute VO2 alteration.

3.2 Work Efficiency

Net efficiency of work during exercise, i.e. theslope of the relationship between VO2 and workrate, appears to be unchanged during pregnancyin non-weight-bearing exercise (e.g. cycle ergo-meter testing).i- '*-'*'''**'! The efficiency of weight-bearing exercise (e.g. treadmill testing), on theother hand, was shown to be improved in earlypregnancy in athletic women who continue ex-ercising and those who stop exercising duringpregnancy.''*' -''"' The increase in exercise efficiencyis obscured after the fifteenth week of pregnancyby pregnancy-associated increases in maternalweight. When adjusted for weight gain, the in-creased efficiency is maintained throughout thepregnancy, with the improvement being greater inwomen who continue exercising during preg-nancy.'^''

3.3 Maximum Aerobic Capacity duringPregnancy

2max' as a criterion measure of cardio-vascular fitness, is poorly documented in preg-nancy. Measuring VO2max during gestation holdsa theoretical risk of inducing fetal stress due toblood distribution favouring maternal skeletalmuscle at the expense of uterine blood flow. Forethical reasons, most studies report estimatedvalues obtained by extrapolating individual sub-maximal heart rate-VO2 curves rather than actualmeasured values at peak exercise intensity. Thefew studies (table I) that directly measuredchanges in maternal VO2max showed no differ-ence in the VO2max (L/min) between pregnant andnonpregnant subjects in cycling,''' ''* - *'"-''- ! swim-ming or weight-bearing exercise.'"*^-^"- '•'''' Wellconditioned women or athletes who maintain amoderate to high level of exercise during andafter pregnancy have even shown a small butsignificant increase in V02niax following preg-

- '*! Thus, pregnancy may have an added

training effect in well conditioned, recreationalsports women.

3,4 Maximal Heart Rate during Pregnancy

Although the V02max values do not seem todiffer significantly in the pregnant compared withthe nonpregnant state, maximal heart rate wasreported to be approximately 4 beats/min lowerin pregnancy compared with post partum.'^"' Theblunted heart rate responses to maximal exercisemay be due to reduced sympathoadrenal res-ponses to heavy exertion during pregnancy.'"*^!

As a result of the increased resting heart rate anddecreased maximal heart rate, heart rate reserve isreduced and heart rate rises to a lesser extent withincreasing VO2, lowering the slope of the heart rate-VO2 relationship during pregnancy compared withthe nonpregnant state.''^-'*'' However, with the ex-ception of resting heart rate, the change in the heartrate-V02 relationship appears not to be afiectedsignificantly by a woman's habitual exercise beha-viour throughout pregnancy.'"'*!

4. Physical Activity Recommendationsduring Pregnancy

In the past, recommendations for physical ac-tivity were based on cultural and traditionalmores rather than scientific evidence. In the1950s, continuation of household chores and a1.6 km (1 mile) walk per day, preferably dividedinto several short sessions, was advised, whereassports and exercise were discouraged.'"'1 In 1985,the American College of Obstetricians and Gyne-cologists (ACOG) formulated one of the firstrecommendations for exercising during preg-nancy. It was advised that the intensity of exerciseshould not induce an increase in heart rate above140 beats/min and that strenuous exercise shouldnot last more than 15 minutes.'-'^' Since then, evi-dence has accumulated on the type, intensity,duration and frequency of exercise beneficial formother and offspring,'^*' leading to the revisionof the guidelines.

Present ACOG recommendations,'^^' and thosejointly published by the Society of Obstetriciansand Gynecologists of Canada (SOGC) and the

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Physieal Activity and Pregnancy 499

Table I. Studies reporting actual measured maximal oxygen uptake (V02max) ¡n pregnant women

Study (year) No. otsubjects

Measurements Findings

Heenan et al.l"' 14(2001)

Lotgering et al.l=°='l 33

(1991, 1992)

McMurray et al.l^^l 10(1991)

Sady et aLl^^l(1988)

45

Sady et al.!""' 45(1989)

Spinnewijn et al.'""'' 11(1996)

VO2 (Umin) measurements in pregnant women(35±0.4wk) and age-matched nonpregnant controlgroup (n = 14) while cycling maximally. Low age-specitic aerobic capacity levels

VO2 measurements (L/min) at 16, 25 and 35 wk otpregnancy and 7 wk post partum at increasing levelsof cycling and treadmill exercise until maximumaerobic power was reached. Average age-specificaerobic capacity levels

V02 measurements (Umin) at 25-35 wk of pregnancyand 9-11 wk post partum during cycling andswimming maximally. Average age-specific aerobiccapacity levels

VO2 (L/min) measurements in pregnant women(26 ± 3 wk) and a nonpregnant control group (n = 10)while cycling maximaily. Low age-specific aerobiccapacity levels

VO2 (Umin) measurements at 26 ± 3 wk of pregnancyand 8 ± 2 wk post partum while cycling maximally. Lowage-specific aerobic capacity ievels

VO2 measurements (L/min) at 30-34 wk of pregnancyand 8-12 wk post partum during cycling andswimming maximaily. Average age-specific aerobiccapacity levels

No significant differences in V02max in pregnantvs control group

No significant differences inpregnancy period vs post partum during cyclingand treadmill exercise

No significant differences inpregnancy period vs post partum during cycling.The swim V02max was significantly greater postpartum than in the 35th swim triais

No significant differences ivs control group

x in pregnant

No significant differences in VO2max inpregnancy period vs post partum

No significant differences in V02max inpregnancy period vs post partum during bicycleand swimming

2 = oxygen uptake.

Canadian Society of Exercise Physiology(CSEP),''**' advise that pregnant women who arefree of medical or obstetric complications followthe American College of Sports Medicine-Centers of Disease Control and Prevention(ACSM-CDC) guidelines for physical activityand exercise. According to these guidelines,pregnant women may safely engage in >30 minutesof moderate physical activity on most, if not all,days of the week.''^' Moderate physical activity isdefined as an activity performed at an intensity ofthree to six metabolic equivalents (METs), whichcorresponds to brisk walking at -5-7 km/h(3-4 mph).''''! Previously sedentary women shouldstart with 15 minutes of continuous exercise threetimes a week, increasing gradually to 30-minutesessions four times a week.' **' The aim of exercisingduring pregnancy is to maintain a good conditionwithout trying to reach a peak fitness level.' **'

Because of the potential risk of certain activ-ities, healthcare professionals should adapt theexercise prescriptions accordingly, prescribing

activities such as walking, swimming, stationarycycling and aquafit rather than gymnastics,horseback riding, skiing, racquet sports or con-tact sports. The risks of injury associated withfalling are increased in latter activities due to in-creased levels of oestrogen and relaxin, whichaugment ligamentous laxity and hypermobi-lj(y [54.60] pg]vic support belts and core stabilityexercise can be used to enable women to remainactive in spite of these changes. In addition,muscle conditioning (light weight-lifting in mod-erate repetitions) is suggested to maintain flex-ibility and muscle tone, prevent gestational lowerback pain, and promote general conditioning.'*'''Abdominal strengthening is difficult to per-form due to the development of diastasis rectiand associated abdominal muscle weakness.''''*'For that reason, pregnant women should avoidexercising in the supine position after ~16 weeksof gestation. Scuba diving is also to be avoidedthroughout pregnancy, as the fetus is not protectedfrom decompression sickness and gas embolism.'- ' '

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500 Melzeret al

Exertion at altitudes above 2500 m (8250 feet) isadvised only after 4-5 days of exposure and accli-matization to such high altitudes.'*^'

Metabolic responses during exercise in preg-nancy are related to the duration and intensityof exercise. Blood glucose of pregnant womendecreases at a faster rate and to a significantlylower level post-exercise than in nonpregnantwomen.'''''1 This decrease does not seem to causehypoglycaemia, even after 40 minutes of moder-ate walking or aerobic dancing.'^''•''^' However,consuming adequate calories and limiting ex-ercise sessions to <45 minutes is advisable.

During the course of pregnancy, thermo-régulation improves as a result of increased bloodcirculation and sweating. Prolonged exercise inhot, humid conditions should nevertheless beavoided and adequate hydration maintainedknowing that maternal core temperature above39.2°C during the first trimester may be poten-tially teratogenic (neural tube defects).'^^' Atpresent, the limited data that are available fromstudies on exercising pregnant women suggestthat women do generally not exercise at levelsthat cause significant

4.1 Recommended Heart Rate Target Zonesfor Aerobic Exercise in Pregnancy

As a result of the reduced maximal heart ratereserve during pregnancy, modified heart ratetarget zones for aerobic exercise are proposed foreach age decade (table II). The proposed mod-ification is based on lowering the top end of thezone by 5 beats/min, thus reducing its range from20 to 15 beats/min.'^'' The heart rate target rangerepresents -60-80% of aerobic capacity of apregnant woman.

Table II. Modified heart rate target zones tor aerobic exercise inpregnancy (reproduced from Davies et ai., '^ ' with permission fromthe Sooiety of Obstetricians and Gynaecoiogists of Canada)

Maternai age(years)

<20

20-29

30-39

>40

i^eart rate targetzone (beats/min)

140-155

135-150

130-145

125-140

iHeart rate target zone(beats/IOsec)

23-26

22-25

21-24

20-23

If the heart rate is chosen to be used solely asan indicator of exercise intensity during preg-nancy, ratings of perceived exertion (RPE) is re-commended to be used in addition to heart rate(table III).' **' RPE consists of numerical ratingsfrom 6-20.'^'^' When multiplied by 10, the RPEratings would roughly correspond to heart rate-observed in healthy young adults. The conven-tional prescriptive zone for fitness training ofhealthy adults is 12-16. Pivarnik et al.'^^' haveshown that this zone is not significantly altered inpregnancy during weight-supported exercise (e.g.cycling). By contrast, the RPE rises in pregnanc>during weight-bearing exercise (e.g. walking)proportionally to increased energy expendituredue to maternal weight gain. For that reason, atarget zone of 12-14 is identified as the recom-mended RPE range in pregnancy.

5. Compiiance with Physical ActivityRecommendations during Pregnancy

Individual physical activity varies greatly du-ring pregnancy, and is determined by socioecono-mic and cultural factors specific to the population.Activity-induced energy expenditure is generallylow in gestation'™"^^' and tends to decrease aspregnancy advances.'^^'^-^' The decrease in physi-cal activity during pregnancy is probably attrib-uted to difficulties of movement related to largerbody mass and discomfort from pregnancy-induced morphological and physiological chan-ges. Reductions also occur as a result of behaviourchanges with respect to the type of activity andin the pace or intensity at which it is carried

Studies reviewing retrospective and prospec-tive data on physical activity during pregnancyconcluded that women decrease physical activityintensity and duration as pregnancy progressesand shift toward performing less intense, morecomfortable modes of activity with lower risks ofmaternal and fetal injury.''*-''* '**'*' Pregnantwomen who ran or jogged before pregnancy werereported to be no longer involved in these modesof exercise but to have shifted toward swimming,walking or gardening. A similar trend can be ob-served regarding occupational activities. Women

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Physieal Activity and Pregnancy 501

Table III. Borg's rating ot perceived exertion' (reproduced fromDavies et al..'^*' with permission from the Society of Obstetriciansand Gynaecologists of Canada)

Borg's rating of perceived exertion

6

7 Very, very light

8

9 Somewhat light

10

11 Fairly light

12

13 Somewhat hard

14

15 Haid

1617 Very hard

18

19 Very, very hard

20a A rating of 12-14 is appropriate for most pregnant women.

with physically strenuous professions are morelikely to stop working during the last trimester ofpregnancy than women working in less physicallydemanding occupations.'**'''

The decrease in activity-related energy ex-penditure during pregnancy can be observed inboth the developed and the developing world.Compared with nonpregnant women, energyexpended on physical activity during pregnancyis decreasing, on average, by 233 kcal/day (20%)at 30 weeks in Sweden,'^*' 201 kcal/day (22%) at25 weeks and 240 kcal/day (23%) at 35 weeks inColombia,'»"! and 103 kcal/day (13%) at 38 weeksin Switzerland.'^"'

As a result, few women reach the recommen-ded physical activity (>30 minutes of moderatephysical activity on most, if not all, days of theweek) during pregnancy. In the US, only 16% ofpregnant women comply with the physical ac-tivity recommendations, compared with 26% ofnonpregnant women.'"' Even healthy, otherwiseactive women decrease physical activity due topregnancy. For example, 70% of women living inSwitzerland complied with the recommendationsduring pregnancy compared with 89% in thenonpregnant state.'"-'

6. Effects of Physical Activity onPregnancy Outcomes

The influence of physical activity on preg-nancy outcomes is widely debated. Traditionally,pregnant women were advised to reduce their lev-els of physical activity. This advice was based onconcerns that exercise would affect pregnancyoutcomes by raising core body temperature, byincreasing the risk of maternal musculoskeletalinjury due to changes in posture, centre of gravityand ligamentous laxity, and by moving transportof oxygen and nutrients to maternal skeletalmuscle rather than to the developing fetus.'^'*'

In the meantime, research has provided a sig-nificant amount of new information about how apregnant woman and her fetus respond to regularengagement in moderate physical activity.'"''New investigations have shown no adversematernal or neonatal outcomes, no abnormal fe-tal growth and no increase in early pregnancy lossor late pregnancy complications.'-^**'

On the contrary, regular physical activity hasproven to result in marked benefits for motherand fetus. Maternal benefits include improvedcardiovascular function, limited pregnancyweight gain,'"'*' decreased musculoskeletal dis-comfort,''' reduced incidence of muscle crampsand lower limb oedema,'**"' mood stability'**^-"''and attenuation of gestational diabetes and ge-stational hypertension.'"''' Fetal benefits includedecreased fat mass, improved stress tolerance,and advanced neurobehavioural maturation.'"^'

The most common fetal response to maternalexercise is alteration in fetal heart rate. The fetalheart rate was reported to increase in the im-mediate post-exercise period, and to be correlatedwith both duration and intensity of maternalexercise.''^"**' Increases in maternal core tempe-rature and maternal-fetal transfer of catechola-mines have been reported as causes of post-exercisefetal tachycardia.'""' Transient decrease in fetalheart rate was also reported during mild ormoderate intensity exercise or in studies involvinghigh intensity maternal exertion.'"^-""' Reduceduterine blood flow caused by a rapid post-exercise fall in Q was postulated as a cause of thebradycardia, although firm physiological bases

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502 Melzer et al.

for these changes are still lacking.'^'l Despite theoverall alterations of fetal heart rate due tomaternal exercise, no abnormal birth outcomeshave been noted in healthy women undergoing anormal pregnancy.''^''!

Only a few studies so far have directly examinedthe effects of physical activity on labour and deliv-ery. One of them was performed by Clapp'"'"' on131 well conditioned recreational athletes. Thosewho continued to exercise at or above 50% of theirpre-pregnancy performance level throughoutpregnancy had a lower incidence of operativeabdominal and vaginal deliveries and fewer fetuseswith acute fetal distress in labour. Erdelyi''"'! re-ported that among 172 Hungarian athletes, thefrequency of caesarean section was almost 50%lower than in a control group of 184 non-athletes.

Even sporadic exercisers, who participatedin a structured exercise programme for at least1 hour, twice weekly for a minimum of 12 weeksduring pregnancy, were more likely to have aspontaneous vaginal delivery than their non-exercising counterparts.''"^' Hall and Kaufmann''"^'studied 845 pregnant women who were given theoption to participate in exercise programmesdesigned specifically for pregnant women. Theyreported that the incidence of caesarean deliverywas 6.7% in the high-exercise group (women whocompleted 64 exercise sessions [range 60-99]during pregnancy), compared with 28.1% in thesedentary group (0.8 sessions [range 0-10] duringpregnancy) [p<0.0001]. Melzer et al.'*^ ' studiedthe effects the recommended levels of >30 minutesof moderate physical activity per day on preg-nancy outcomes in 44 healthy Swiss women. Ac-tive women had a lower risk of operative deliverycompared with inactive women (odds ratio =3.67; 95% CI 1.02, 13.1). Adjustment for parity,maternal weight gain and infant weightstrengthened the association between lack ofphysical activity and operative delivery (adjustedodds ratio = 7.65; 95% CI 1.27, 45.84).

Prior studies showed a strong correlationbetween length of labour and physical fit-ness [100-102,104.105] Penttinen and Erkkola,"»"' onthe other hand, found no significant differencesin labour parameters between athletes and con-trols. Many other studies''"^-'"^'"**' also reported

no significant difference in labour duration be-tween recreationally active women and theircontrols. Lastly, maternal exercise has been re-ported to increase,'"^-^-'"'^-""' decrease''""' or haveno effect'^'*''"^'"'-"^' on newborn birthweight.

The inconsistent results on the effects of ex-ercise on the newborn's birthweight may bedue to the differences in type, timing, frequencyand duration of the exercise regimen imposed(table IV). Clapp et al."'^' demonstrated thatwomen who begin a regular, moderate regimen ofweight-bearing exercise (treadmill, step aerobicor stair stepper) in early pregnancy (for 20 minutes3-5 times per week) gave birth to larger newbornbirthweights compared with non-exercising mo-thers. Other studies further indicated that womenwho stayed active throughout the pregnancy alsoexperienced improved fetal growth.'"'-'"'"^""'Physical activity during pregnancy may thus bean important mechanism for improving placentalfunctional capacity, circulation and gas exchange,which in turn increases nutrient dehvery andoverall growth rate of the fetus.'"'''"-^'

Nevertheless, weight-bearing exercise wasshown to influence fetal growth in both a time-dependent and exercise volume-dependent fash-ion.'""'^ Women who increased the volume oftheir moderate weight-bearing exercise (tread-mill, step aerobic or stair stepper) in late preg-nancy (20 minutes 3-5 times per week throughweek 20, gradually increasing to 60 minutes5 days per week by week 24 and maintaining thatregimen until delivery) experienced a significantlylower newborn birthweight than those womenwho maintained the high volume of exercise inearly pregnancy and then decreased it by two-thirds in late pregnancy.'"'*' For women who ex-ercised to term, the drop in infant weight wasattributed mainly to differences in newborn fatmass. Clapp and Capeless'"*^' concluded thatmost of the variabihty in birthweight (40%) canbe explained by the relative level of weight-supported exercise performance in the last 5 monthsof pregnancy. Other studies have been unable todemonstrate decreased birthweight in womenwho continue exercising but at reduced levels, orin those performing weight-supported exercise(cycling) and/or weight-lifting exercise.''"^-'"'''

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Physical Activity and Pregnancy 503

Table IV. Effect of a strijctured exercise programme during pregnancy on a newborn's birthweight

Study (year) Number of subjects Exercise mode Newborn birthweight

Beci<mann and 50 exercise group; 50 control group

87 exercise group (runners, aerobicdancers); 44 control group (runners,aerobic dancers who stopped exercisingduring pregnancy)

(1990)

Clappl'™'(1990)

Exercise group participated in a non-endurance strength training programmefor at least 1 h, 2x/wk, for a minimum of12wk during pregnancyExercise at >50% of preconception level(preconception level; running,14-68 km/wk, 51-83% VOs^g^; aerobics,3-11 sessions/wk, 25-30 min at 54-90%

No statistical difference inbirthweight between exercise andcontrol group (3,69 ±0,3 vs3,65 ±0.3 kg)

Exercise group had heavier babiesthan those born to controi groupwomen (3,78 ±0,4 vs 3,37 ±0,3 kg;p = 0,01)

Clapp et al,'^'l 34 exercise group; 31 control group(1999)

Clapp et a l , ' " ' ' 22 exercise group; 24 control group(2000)

Clapp et a l , ' " " ' 26 L-H; 24 M-M; 25 H-L(2002)

Collingset al,l'°^l(1983)

Hail andKaufman(1987)

Kardel andKasel"21

(1998)

Controi group had heavier babiesthan those born to exercise groupwomen (3,64±0,05vs3,44±0,1 kg;p = 0,01)Exercising mothers had heavierbabies than those born to controlwomen (3,75 ± 0,8 vs 3,49 ±0,7 kg;p = 0,05)

H-L group had heavier babiescompared with the M-M and H-Lgroups (3,90±0,1 vs 3,44±0,1 vs3,34±0.1kg;p<0.001)

Throughout pregnancy: running,aerobics, swimming or stair-climbing,3x/wk, >20 min, >55% VOa^ax

Weight-bearing exercise (treadmill, stepaerobics or stair stepper) from the 8th wkof pregnancy, 20min, 3-5x/wk at55-60% pre-pregnancy VOamaxFrom the 8th wk of pregnancy, at55-60% V02max: L-H, 20min, 5x/wkthrough wk 20, increasing to 60 min5x/wk by wk 24 and maintaining thatlevel until delivery; M-M, 40 min, 5x/wk;H-L, 60 min, 5x/wk through wk 20,decreasing to 20 min, 5x/wk by wk 24and maintaining that level until delivery

During the 2nd and 3rd trimester ofpregnancy; aerobic exercise 3x/wk,>25 min, 65-70% VOs^^,

Throughout pregnancy, exercise sessionconsisted of 3 components; warm-up(5 min treadmill at 5-6 km/h, or 5 minbicycle at 50 W) + 5x/wk, 45 min weight-iifting + bicycie ergometer (-85% ofmaximal HR but <140 beats/min)From 20th wk of pregnancy, 3-partprogramme;1, Muscie strength; MEG and HEG;5x/wk, 72 min .2, Intervai aerobic(HR = 170-180 beats/min); MEG, tstday15 sec exercise, 15 sec rest for 10 min,2 times with 5 min break between; HEG,the same as MEG with 15 min rest3, Endurance aerobic (HR120-140 beats/min); MEG and HEG,2x/wk, 90 min

HEG = high intensity exercise group; H-L = high-low exercise group; HR = heart rate; L-H = iow-high exercise group; MEG = medium intensityexercise group; M-M = moderate-moderate exercise group; VO;„,ax = maximal oxygen uptake,

20 exercise group; 12 control group

393 control group (average 0,8sessions); 82 iow level exercise group(15 sessions); 309 medium levelexercise group (32 sessions); 61 highlevel exercise group (64 sessions)

21 MEG; 21 HEG

Exercise group had no significantlyheavier babies from those born tocontrol group women (3,60 ±0,4 vs3,35 ±0,4 kg)

High exercise group had 151 gheavier babies than those born tocontrol group women (3510 vs3359 g;p = 0,06)

No statistical difference inbirthweight between MEG andHEG group (3,59 ± 0,5 vs3,65 ±0,5 kg)

7. Conclusions

Various physiological modifications arise dur-ing pregnancy, labour and delivery in order to

meet the increased metabolic demands of motherand fetus. Despite the pregnancy-induced physio-logical changes, moderate physical activity dur-ing normal pregnancy appears to be beneficial

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r

504 Melzer et ai

and should be encouraged. Unfortunately, en-ergy expended on physical activity often de-creases as pregnancy advances, which may havenegative consequences on mother and infant,including maternal cardiovascular fitness andcapacity to sustain efforts required during labourand delivery. Further studies with larger samplesizes are required to confirm the association be-tween physical activity and outcomes of labourand delivery.

Acknowledgements

The study was supported in part by a competitive grantattributed to Drs Boulvain and Kayser by the Clinical Re-search Centre, Faculty of Medicine, University of Geneva,and was further supported by the Faculty of Medicine ofthe University of Geneva, and the University Hospitals ofGeneva, Switzerland. The SOGC guideline has been providedfree of charge courtesy of the Society of Obstetricians andGynecologists of Canada. The authors have no conflicts ofinterest that are directly relevant to the content of this review.

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Correspondence: Professor Bengt Kayser, Director, Instituteof Movement Sciences and Sports Medicine, Faculté deMédecine, Université de Genève, 10 rue du Conseil Gén-éral, 1205 Genève, Switzerland.E-mail: bengt.kayser@unige.ch

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