Physical activity and maternal obesity: cardiovascularadaptations, exercise recommendations,and pregnancy outcomes

Michelle F Mottola

Although a healthy lifestyle approach is intuitive for obese pregnant women, noguidelines currently exist to manage these women throughout pregnancy. Womenwho are medically prescreened for contraindications can engage in a walkingprogram three to four times per week, starting at 25 min per session and adding2 min per week until reaching 40 min, with sessions continuing until delivery. Atarget heart rate of 102–124 beats per minute should be promoted for women20–29 years of age and a rate of 101–120 beats per minute for women 30–39 yearsof age. A pedometer step count of 10,000 steps per day is suggested as a goal, as thislevel of activity provides important health benefits. Combining healthy eating with awalking plan prevents excessive weight gain during pregnancy and promotes ahealthy fetal environment.© 2013 International Life Sciences Institute

INTRODUCTION

Women of childbearing age are at a greater risk ofobesity because excessive weight may be gained duringpregnancy and retained after delivery. Guidelinesrecommend that obese (prepregnancy body mass index[BMI] ≥30.0 kg/m2) women gain between 11 lb and 20 lb(5–9 kg) during pregnancy, at a rate of weight gain of0.4–0.6 lb (0.2–0.3 kg) per week during the second andthird trimesters, assuming an initial weight gain of up to4.4 lb (2 kg).1 Prevention of excessive weight gain duringpregnancy is highly recommended to reduce the risk ofobesity, gestational diabetes mellitus, type 2 diabetes,hypertension, and cardiovascular disease in women ofchildbearing age.2 The lifestyle that leads to obesity andexcessive weight gain is often marked by an unhealthydiet and a lack of physical activity. Obesity has a directeffect on indicators of health and chronic disease risk, notonly for the pregnant woman but also the developingfetus.3,4 Although the practice of a healthy lifestyle and theprevention of excessive weight gain are both important

during pregnancy, there are currently no recommenda-tions on nutrition and physical activity to guide health-care workers in the management of the obese pregnantwoman. This article discusses cardiorespiratory adapta-tions during pregnancy, exercise, and obesity, and pro-vides exercise guidelines and recommendations for ahealthy lifestyle approach for obese pregnant women,including a discussion of pregnancy outcome.

CARDIORESPIRATORY CHANGES DURING PREGNANCYAND RESPONSES TO EXERCISE

Pregnancy may offer protection from cardiovasculardisease in women at low risk of obstetric complications,as evidence suggests that the maternal cardiovascularsystem is remodeled in early gestation by an estrogen-mediated reduction in vascular tone, which leads to aprimary reduction in afterload and an increase in venouscapacitance,5 reflected in increased resting cardiac outputof about 50% over nonpregnant values.6 An increase inventricular cavity dimension without an increase in wall

Affiliation: MF Mottola is with the R. Samuel McLaughlin Foundation – Exercise & Pregnancy Laboratory, School of Kinesiology, Faculty ofHealth Sciences, and the Department of Anatomy, Schulich School of Medicine & Dentistry, The University of Western Ontario, London,Ontario, Canada.

Correspondence: MF Mottola, R. Samuel McLaughlin Foundation – Exercise & Pregnancy Laboratory, The University of Western Ontario,London, Ontario, Canada N6A 3K7. E-mail: mmottola@uwo.ca. Phone: +1-519-661-2111, ext. 88366. Fax: +1-519-661-2008.

Key words: exercise prescription, healthy lifestyle, obese pregnant women, walking

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Supplement Article

doi:10.1111/nure.12064Nutrition Reviews® Vol. 71(Suppl. 1):S31–S36 S31

thickness,7 an increase in aortic capacitance,8 and a reduc-tion in peripheral vascular resistance all occur around thesame time.9 In addition, the early pregnancy-inducedchanges in cardiac output are thought to occur inresponse to an increase in resting heart rate (HR), as mostof the 15–20 beat increase in HR over nonpregnant valuesoccurs in the first trimester.10 Stroke volume alsoincreases by approximately 10% at the end of the firsttrimester11 and occurs before significant enhancement inmaternal blood volume,5 which may increase by up to50% above nonpregnant values by late pregnancy.12

Pregnancy-induced hormones that reduce peripheralvascular resistance also activate the renin-angiotensinsystem, leading to increased secretion of an antidiuretichormone (arginine vasopressin) to retain fluid and main-tain or slightly reduce blood pressure.5

Similarly,there are pregnancy-induced adaptations tothe maternal respiratory system as remodeling and expan-sion of the thoracic cage occur, leading to a higher dia-phragmatic midposition,13 which results in a reduction inresidual volume and expiratory reserve volume. This leadsto an increase in inspiratory capacity, though the effect onvital capacity is minimal.14 One of the most substantialphysiological pregnancy-induced changes, which possiblyserves to protect the fetus, is an increase in respiratorysensitivity to carbon dioxide.This change is observed earlyin pregnancy and causes an increase in tidal volume andminute ventilation, leading to a reduction in arterialcarbon dioxide tension and an augmentation in arterialoxygen tension.15 These changes create a buffer zone, pos-sibly to protect the fetus from acute elevations in maternalcarbon dioxide,16 and the early increase in maternalminute ventilation may prevent fetal hypercapnia andacidosis throughout pregnancy.17

Many healthy pregnant women complain ofrespiratory discomfort (dyspnea), especially in latepregnancy, both at rest and after exertion.18 Perceptionsof respiratory effort and dyspnea appear to be reducedduring submaximal steady-state exercise throughout ges-tation.19 It may be that the maternal anatomical andmechanical adaptations of the respiratory system reduceairway resistance, preserve breathing mechanics, andminimize the effort of ventilation, thereby reducingdyspnea with the concomitant increase in minute venti-lation during exercise.20 Resting oxygen uptake(expressed as mL/kg/min) reflects the increase in bodymass during pregnancy and thus declines slightly duringeach trimester.21

Pregnancy and aerobic conditioning are biologicalprocesses that involve striking physiological adaptationsthat may occur in the same direction or in opposite direc-tions, depending on the specific variable being studied.21

Although absolute oxygen uptake is well preserved inwomen who maintain physical activity, maximum exer-

cise stress testing is not recommended during pregnancy.Functional cardiac reserve (maximum HR minus restingHR) is decreased during pregnancy because of elevatedresting HR, and the magnitude of heart rate reserve(HRR) is also decreased compared with nonpregnantvalues.21 Because maternal HR increases at a slower ratein response to increases in exercise intensity, target HRzones for exercise prescription must be derived frompregnant women. The efficiency of standard submaximalexercise for body-weight-supported exercise, such ascycling, does not change during pregnancy, yet forweight-bearing exercise, such as walking, the energyrequirement increases in proportion to the maternalmass gain.22

OBESITY AND CARDIORESPIRATORY ALTERATIONS

Obesity may influence pregnancy by causing majorhealth risks to both the mother and the fetus, as obesepregnant women are at increased risk of cardiovasculardisease, gestational diabetes, and delivering large- orsmall-for-gestational-age infants. Moreover, obesewomen may have more complications during pregnancyand birth.3 In addition, recent evidence indicates that thefetal environment, especially as related to maternalprepregnancy BMI, and excessive gestational weight gainboth increase risk factors for future chronic disease in theoffspring.4 Obesity in the nonpregnant individual affectsevery major organ system, lowering the work rate(amount of work done at a given HR) and oxygen uptakeand independently decreasing the mechanical efficiencyof breathing as a result of both the increased deposition ofadipose tissue on the chest and abdomen and thedecreased compliance of the ribcage, resulting in thecharacteristic rapid shallow breathing.23 Obesity alsoincreases metabolic cost due to the increase in the energyneeded to move larger limbs, the increased work ofbreathing, and a decreased peripheral motor efficiency.20

Cardiorespiratory responses to and work efficiency ofgraded treadmill exercise in healthy nonpregnant women(n = 14), in normal-weight (prepregnant BMI 18.9–24.9 kg/m2) pregnant women (n = 20), and in obese(prepregnant BMI ≥ 30.0 kg/m2) pregnant women (n =20) matched for age were compared to assess the effects ofobesity on pregnant women at 16–20 weeks of gestationwhile exercising.24 Treadmill exercise was chosen insteadof bike exercise to simulate normal daily living tasks.The results showed that exercise duration and peaktreadmill speed were lower in pregnant normal-weightwomen (23.9 ± 4.9 min; 1.6 ± 0.2 m/s; P < 0.01) than innonpregnant women (33.7 ± 4.9 min; 2.0 m/s) and werefurther reduced in pregnant obese women (19.6 ±2.8 min; 1.4 ± 0.1 m/s; P < 0.01), indicating a limitation toperforming exercise, although HR and work rate were not

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significantly different between groups.24 In addition, theventilatory response both at rest and to exercise increasesduring pregnancy and is further augmented by obesity.However, contrary to the ventilatory response, the normalincrease in HR and the concurrent diminished HRR seenin normal-weight pregnant women during exercisewas not further affected by obesity at standardizedsubmaximal exercise levels of 50 (mild intensity) and 100(moderate intensity) watts.24 It was concluded that healthyobese pregnant women have the aerobic capacity to under-take structured walking activities at standardizedsubmaximal levels,which lends support to the feasibility ofexercise prescription for this population group.25

EXERCISE GUIDELINES FOR OBESE PREGNANT WOMEN

In Canada, a medical prescreening tool called the PAR-med-X for Pregnancy26 can be used by healthcare provid-ers to screen for contraindications to exercise and toprovide guidelines for exercise prescription based on theFITT (frequency, intensity, time, and type of activity)principle.26 It provides target HR ranges, based on age,that were validated in pregnant normal-weight women atan intensity of 60–80% of maximum oxygen capacity27 tomonitor intensity. For overweight and obese pregnantwomen who are medically prescreened, this intensity maybe too strong and may prevent them from exercising. TheAmerican College of Sports Medicine28 suggested thatpreviously sedentary overweight and obese pregnantwomen should start an aerobic exercise program at anintensity equivalent to 20–39% of maximum aerobic(heart and lung) capacity, which indicates the lowest levelof physical activity that would provide health benefits.28

Using a graded treadmill exercise test in 106 pregnantoverweight (prepregnancy BMI ≥ 25.0–29.9 kg/m2) andobese (prepregnancy BMI ≥ 30.0 kg/m2) women, targetHR zones based on age were validated at the lower exer-cise intensity suggested by the American College ofSports Medicine.29 Based on the results, target HR zonesof 102–124 beats per minute (bpm) for women 20–29years of age and 101–120 bpm for women 30–39 yearsof age were suggested for use in exercise intensity pre-scription in medically prescreened, previously sedentaryoverweight and obese pregnant women.29

Walking appears to be the most popular activity forpregnant women.30 The frequency of other types of physi-cal activity decreases as pregnancy progresses, but thefrequency of walking has been shown to increase.30 Theseobservations suggest that walking is an important aerobicactivity that should not be ignored, as it mayhelp maintain aerobic fitness in pregnant women.Ruchat et al.31 investigated the effect of a maternalwalking program of low intensity (30% HRR) or vigorousintensity (70% HRR) on cardiorespiratory responses to

standard submaximal treadmill exercise in normal-weight(prepregnancy BMI 18.5–24.9 kg/m2) women beforeand after an 18-week intervention, starting at 16–20weeks of pregnancy. The results suggested that both exer-cise intensity groups experienced an improvement insubmaximal aerobic capacity, with greater improvementobserved in the vigorous-intensity group. However,women in both groups presented similar gestationalweight gain, and all delivered healthy babies, showing thatprenatal walking of low or vigorous intensity, combinedwith healthy eating, is an important component of ahealthy pregnancy in normal-weight women.31 It wouldseem reasonable to suggest that walking, combined with ahealthy dietary plan, would also be beneficial in obesepregnant women at low obstetric risk.

The American College of Obstetricians and Gyne-cologists32 suggests that medically prescreened pregnantwomen can exercise on most if not all days of the week,while the latest guidelines for Americans suggest that exer-cise be spread throughout the week.33 However, the fre-quency of structured exercise,especially in late pregnancy,was found to be a determinant of birth weight in a case-controlled study of 526 women.The odds of giving birth toa small-for-gestational age baby was 4.6 times more likelyfor women who engaged in structured exercise more thanfive times per week and 2.6 times more likely for womenwho engaged in structured exercise two or fewer times perweek, regardless of exercise intensity or duration of activ-ity.34 Small-for-gestational-age babies are at risk forobesity and cardiovascular disease later in life.4 Structuredexercise performed three to four times per week,with a dayof rest between each exercise day, would seem ideal andmay also help reduce fatigue.26

Overweight and obese pregnant women who, aftermedical prescreening, wish to start a structured walkingprogram should initially attempt 25 min per session,adding 2 min per session each week until reaching 40 minper session,35 which can be maintained until delivery,even if it is necessary to reduce the intensity or to includerest intervals.25 The best time to progress is in the secondtrimester, when the risks and discomforts of pregnancyare lowest.25,26 Another way to confirm the appropriateintensity is to use the “talk test,” which indicates that theintensity is appropriate if an obese pregnant woman cancarry on a conversation while walking.26

HEALTHY LIFESTYLE APPROACH FOR OBESEPREGNANT WOMEN

Although a healthy lifestyle approach is intuitive forobese pregnant women, no guidelines currently exist tohelp manage these women throughout their pregnancy.Gestational weight gain guidelines exist that suggestwomen who are classified as obese before pregnancy

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(BMI ≥ 30.0 kg/m2) should gain between 11 lb and 20 lb(5–9 kg) at a rate of weight gain of 0.4–0.6 lb (0.2–0.3 kg)per week during the second and third trimesters, assum-ing an initial weight gain of up to 4.4 lb (2 kg).1 Excessiveweight gain above the recommended range for pregnantwomen may result in excess fat stores along with theassociated health risks for mother and fetus.36

A healthy lifestyle is a balance between eatingnutritious foods (not “eating for two” but eating twiceas healthy), watching portion sizes, and being physicallyactive.36 A recent study examined the effects of aNutrition and Exercise Lifestyle Intervention Program(NELIP) on the prevention of excessive weight gain inoverweight (prepregnancy BMI ≥ 25.0–29.9 kg/m2) andobese (prepregnancy BMI ≥ 30.0 kg/m2) pregnantwomen. The goals of the nutrition component of NELIPwere to individualize the total energy intake as follows: 1)energy intake of approximately 2,000 kcal/day (8,360 kJ/day), with a restriction not exceeding 33% of total energyintake; 2) total carbohydrate intake of 40–55% of totalenergy intake, with carbohydrate intake distributedthroughout the day; 3) three balanced meals and three tofour snacks per day, emphasizing complex carbohydratesand low-glycemic-index foods; 4) total fat intake individu-alized to 30% of total energy intake (substitutingmonounsaturated fatty acids for saturated and trans-fattyacids); 5) protein intake of 20–30% of energy; and 6)micronutrient and fluid intakes in accordance with thoserecommended for pregnant women.35 The nutritioncomponent was based on medical nutrition therapy givento women with gestational diabetes. The exercisecomponent of NELIP was based on previous work29 andconsisted of a mild walking program (30% of HRR)to facilitate compliance. All women were medicallyprescreened using the PARmed-X for Pregnancy,26 and allstarted the program between 16 weeks and 20 weeks ofgestation. The exercise program began with 25 min ofwalking per session, three to four times per week. Eachsubsequent week thereafter, the exercise time increased by2 min, until a maximum of 40 min was reached andmaintained until delivery.35 Pedometers were worn tocount steps, and steps were recorded in exercise logs tomonitor activity.35

Comparison of preintervention (16–20 weeks of ges-tation) with postintervention (34–36 weeks of gestation)data showed that NELIP participants decreased theirmean daily total energy intake from 2,228.0 ± 474.6 kcalto 1,900.2 ± 343 kcal, and daily carbohydrate intakedropped from 318.5 ± 155.1 g to 259.1 ± 93.9 g, while thepercentage of daily energy from protein increased from16.9 ± 2.4% to 18.4 ± 2.3% (P < 0.05). The average dailypedometer step count before the intervention programwas 5,677.6 ± 1,738.0 steps. The step counts at 25 min(length of initial exercise session) were, on average,

2,861 ± 287.7 steps, which increased to 4,406.9 ± 461.0steps at 40 min per session. When mean daily steps wereadded to the steps taken at the end of the program (40 minof structured walking), the women were taking more than10,000 steps, which brought them from a preinterventionrating of “low active” or “sedentary” on the activity indexused to“active” at the end of the program.37 Maintaining aphysical activity index rating of“active” or above is recom-mended to achieve a healthy lifestyle in the nonpregnantpopulation.37 In addition, the average HR of the NELIPparticipants was 118 ± 6.8 bpm, which was within therange for aerobic benefits to occur.31

In terms of birth outcome, the women in the NELIPprogram were compared with a cohort of womenmatched by prepregnancy BMI, maternal age, and parity(matched 4 to 1). Infant birth weight and gestational ageat delivery did not differ between the NELIP and thematched control women. However, when stratified byBMI, 3.2% of the overweight NELIP women had babiesweighing between 4.0 kg and 4.5 kg, compared with 18%of the matched controls (P = 0.048). No babies born to theNELIP women weighed less than 2.5 kg, whereas 3.5% ofthe babies born to the matched controls had birth weightsbelow 2.5 kg. The rate of cesarean delivery was similar inboth groups: 4.6% among the NELIP women and 7%among the matched controls. In addition, 3% of NELIPwomen developed gestational diabetes mellitus (noinsulin required), compared with 7.8% (with 55% of thoseneeding insulin) of those in the matched control group.35

Excessive weight gain was successfully prevented in80% of the NELIP women, many of whom had difficultywith weight loss programs in the year before the currentpregnancy.36 This is an important finding because manyof the multiparous women (84%) had experienced exces-sive weight retention (10.3 kg) from previous pregnan-cies. In addition, before taking part in the NELIPprogram, many participants had already experiencedexcessive weight gain (4.5 kg) that was substantially morethan the suggested guideline of 2 kg for women in the firsttrimester.1 However, while in NELIP, subsequent weightgain was on average 6.8 ± 4.1 kg (0.38 ± 0.2 kg/week),with a total pregnancy weight gain of 12.0 ± 5.7 kg andexcessive weight gain occurring before NELIP began, at16 weeks of gestation.35

Excessive weight gain during pregnancy is animportant issue that needs further examination, especiallywith regard to pregnancy outcome. Davenport et al.38

examined whether timing of excessive weight gain, before16 weeks (usual time for the NELIP intervention to beinitiated), after 16 weeks (i.e., during the intervention), orat both time points, was important when assessing birthweight and body fatness of newborns at delivery. In acohort of 172 women,33.7% of whom were normal weight(prepregnancy BMI ≥ 18.5–24.9 kg/m2), 33.7% of whom

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were overweight (prepregnancy BMI ≥ 25.0–29.9 kg/m2),and 32.6% of whom were classified as obese(prepregnancy BMI ≥ 30.0 kg/m2), all women wereinitiated into the NELIP between 16 weeks and 20 weeksof gestation. Weight gain was assessed before theintervention (up to 16–20 weeks of gestation) and fromthe start of the intervention to delivery (after 16–20 weeksof gestation). Women were stratified, based onprepregnancy BMI category, as follows: 1) appropriategestational weight gain that was within the Institute ofMedicine recommendations1 for the first and secondhalves of pregnancy (overall appropriate); 2) appropriategestational weight gain in the first half of pregnancy, butexcessive gestational weight gain in the second half ofpregnancy (late excessive); 3) excessive gestational weightgain in the first half of pregnancy, but appropriategestational weight gain in the second half of pregnancy(early excessive); and 4) excessive gestational weightgain throughout pregnancy (overall excessive). Infantbirth weight and adiposity (according to the Catalanoet al.39 equation) were compared between groups. Aftercontrolling for maternal prepregnancy BMI,maternal age,infant gestational age at delivery, and gender of the infant,it was found that those women who gained excessivelyprior to the intervention and then gained within theInstitute of Medicine guidelines1 (early excessive) andthose women who gained excessively throughoutpregnancy (overall excessive) gave birth to babies withexcess normative infant body fat.38 In addition, the timingof excessive maternal weight gain, specifically during thefirst half of pregnancy, was a stronger predictor of infantbody fat at birth than total maternal weight gain,regardlessof prepregnancy BMI.38 These preliminary results warrantfurther research on the timing of healthy lifestyleinterventions and the impact of such interventions onprevention of excessive gestational weight gain andpregnancy outcomes.

CONCLUSION

Women of childbearing age are at a greater risk of obesitybecause excessive weight may be gained during preg-nancy and retained after delivery. Therefore, adherence torecommendations about the amount of weight a womanshould gain during pregnancy (with particular attentionto BMI status) may prevent chronic disease risks for amother and her offspring. Currently, there are no recom-mendations on nutrition and physical activity to guidehealthcare workers in the management of obese pregnantwomen. Healthy obese pregnant women have the aerobiccapacity to undertake structured walking activities,which lends support to the feasibility of exerciseprescription for this population group. By adopting theFITT principle of exercise prescription, it is suggested that

obese pregnant women who are medically prescreened forcontraindications to exercise can engage in physical activ-ity three to four times per week (Frequency),using a targetHR range of 102–124 bpm for women 20–29 years of ageand a range of 101–120 bpm for women 30–39 years of agewhile maintaining the ability to carry on a conversation(intensity), starting with 25 min per session and adding2 min per week until sessions reach 40 min (time persession), and continuing until delivery. Walking (type) isthe most popular activity among pregnant women and canbe monitored by using pedometer step counts, which canbe a great motivator.An aim of approximately 10,000 stepsper day is suggested in order to obtain health benefits.Combining healthy eating with a walking plan will helpprevent excessive weight gain and promote a healthy fetalenvironment and pregnancy outcome. Further research isnecessary to determine the optimal timing of initiatinghealthy lifestyle interventions to prevent excessive gesta-tional weight gain in obese pregnant women and the sub-sequent impact on infant health and body fatness.

Acknowledgments

Funding. Funding provided by the Canadian Institutes ofHealth Research (CIHR) and the Rx&D Health ResearchFoundation of Canada.

Declaration of interest. The authors have no relevantinterests to declare.

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