The physiological changes that occur during pregnancy cause an increased demand for dietary energy as a result of increased oxygen consumption, respiration, circulation and renal function of the foetus during development [33]. From conception to birth, all the growth of the foetus is possible because of the nutrients the mother consumes [34]. The nutrient needs during pregnancy and lactation are higher than any other time in a woman’s life [34]. This high nutrient demand during pregnancy is met with an increased energy intake, as well as help from the mother’s body that maximises absorption and minimises energy expenditure [34].

The energy needs of pregnant women exceed those of nonpregnant women by an additional 340 kilocalories per day during the second trimester and extra 450 kilocalories per day during the third trimester [34]. The additional kilocalories represent 15–20 % more food than before pregnancy for an average 2000-kilocalorie daily intake. Ample carbohydrates are essential for fuel to the foetal brain, which ensures that the protein needed for growth is catabolised and used to synthesise glucose [34]. The extra energy demands of pregnancy can be met by an increase in food intake or by the mobilisation of energy fat stores of the mother, particularly those mothers with sufficient energy reserves [35].

The additional energy requirements during pregnancy can be described as the energy needed for maternal tissue and foetal growth , as well as the energy required for the rise in basal metabolic rate and the changes in physical activity [35]. Energy requirements during pregnancy remain controversial because of conflicting data on maternal fat deposition and putative reductions in the mother’s physical activity as the pregnancy advances [36].

Total daily energy expenditure (TDEE) consists of three general factors: resting metabolic rate, thermogenic effect of feeding and physical activity [34, 37]. TDEE for the nonpregnant healthy woman is calculated as the energy expended on resting metabolic rate (60–75 %), thermogenic effect of feeding (10 %) and physical activity (25–30 %). TDEE increases during pregnancy because of tissue growth, an elevated basal metabolic rate and the increased energy costs of moving a heavier body [38].

Resting metabolic rate (RMR) accounts for all the metabolic activities in the human body [34]. Human metabolism involves all the body’s chemical reactions of biomolecules that cause anabolism and catabolism. RMR varies dramatically from person to person and for the same individual with a change in circumstances or physical condition (with pregnancy being an extreme physiological condition) [34]. Pregnancy is a dynamic, anabolic state where the human body obtains energy for growth and maintenance [39].

The enhanced work of pregnancy raises the RMR dramatically and demands extra energy [40, 41]. This is calculated by Prentice et al. [42] as 20 % in late pregnancy. Forty percent of this variability is explained by the percentage of total body fat before pregnancy and the gain in body weight during pregnancy [35, 41]. Body fat gain accounts for about 55.5 ± 20 % of total weight gain during pregnancy [43]. According to Löf et al. [41], factors that are responsible for the variability in RMR response during pregnancy differ in the earlier and later trimesters of pregnancy . Most of the total body fat mass is deposited during the second trimester, with little change taking place in the first and third [44]. Chamberlain and Popkin [33] developed a theoretical model to estimate energy requirements during pregnancy (Fig. 16.2) [45], assuming an average gestational weight gain (GWG) of 12.5 kg (≈ 0.925 kg protein, ≈3.8 kg fat, and ≈ 7.8 kg water), which is associated with an increase in RMR [41].

The thermogenic effect of food is attributed to the digestion process and the energy cost of storage of the exogenous macronutrient is proportional to the food energy that is consumed [34]. This diet-induced thermogenesis seems to be unaltered [36, 42, 46–49] or even reduced [44, 50, 51] during pregnancy.

The most varying factor that determines total energy expenditure is physical activity and is dependent on three factors: muscle mass, body weight and level of activity [34]. The interaction between physical activity and energy metabolism is complex. For example, pregnant women may reduce physical activity energy expenditure by selecting less demanding activities or reducing the pace of activity, although the actual cost might be higher, because of moving a heavier body [38]. However, all pregnant women might not reduce their physical activity because of the knowledge they have of the health benefits of regular physical activity during pregnancy. Over the past 2 years, more studies have focused on the energy expenditure during pregnancy, especially in the wake of the rapid increase in obesity, globally. The measurement of total energy expenditure during pregnancy is controversial, mainly because of conflicting data on the extent of reduction in physical activity as pregnancy advances [35] and the collection of physical activity information with self-report questionnaires.

The energy cost that is attributed to physical activity during pregnancy is generally lower [40, 52–55] and tends to decrease as pregnancy advances [56–60]. Studies show that pregnant Scottish [61] and Dutch [60] women had a slight decrease in absolute energy cost of physical activity , observed in activity diary studies, as their pregnancy advanced. The same results were found in British women by Prentice et al. [42] by means of whole body indirect calorimetry methodology. However, Melzer et al. [35] found this decrease in active energy expenditure insignificant in pregnant women in Sweden and America, but when expressed as per unit of body weight to account for weight differences, this result became significant. Other studies from Sweden and the UK report similar decreases in active energy expenditure per kilogram in the pregnant compared to the nonpregnant state [42, 54]. Preliminary findings of the Habitual Activity Patterns during PregnancY (HAPPY)-study in Potchefstroom, South Africa, indicate a 25 % reduction in activity energy expenditure from the first to the third trimester of pregnancy. The study included participants from white, black and coloured ethnic groups as well as low-, middle- and high socioeconomic groups [62]. The physical activity levels (PAL) reported can be classified as low activity to sedentary behaviour from the first to the third trimester of pregnancy.

Reasons for this decrease in physical activity are explained in the following section. However, physical activity cannot be observed in isolation when activity energy expenditure is discussed because the energy intake is also important in the energy balance . More details regarding behavioural changes in activity patterns are discussed in the following section.

To better understand the associations between energy intake, energy storage and energy expenditure during pregnancy, studies should be carried out during free-living conditions applying the most objective and reliable methodology [38]. The correct measuring tool is essential to quantify physical activity during pregnancy.

A great variety of physical activity questionnaires have been developed and validated over the past 20 years. The accuracy of self-reporting questionnaires is influenced by the subjective nature of the term “intensity of physical activity” [63]. Physical activity questionnaires emphasise participation in moderate to vigorous sports while not including household or childcare activity [64]. Indeed, women spend considerable time and energy in moderate intensity activities related to household chores, their job and family care [65]. Interestingly, the accuracy of short- and long-term recollections of physical activity patterns by pregnant women is not known [66]. According to Poudevigne and O’Conner [66] there is a lack of knowledge regarding how accurately women can recall their physical activity patterns during pregnancy.

Direct measurements of the metabolic cost of energy expenditure among pregnant women, as opposed to relying upon values collected among nonpregnant populations, will objectively define the intensity of recreational activity among pregnant women [3]. For this purpose, double-labelled water and indirect calorimetry [67, 68] are used to measure physical activity, but because of the costs, invasiveness and technical sophistication of these methods, their suitability for the general population decreases.

In large samples and population-based studies, questionnaires have been the instrument of choice. Hermann et al. [69] determined the validity of two questionnaires, namely the International Physical Activity Questionnaire (IPAQ) [70, 71] and the Global Physical Activity Questionnaire version 2 (GPAQ) [72]. The GPAQ shows short- and long-term retest reliability and modest validity [69], although it has not been validated in the pregnant population. Specifically during pregnancy , four validated questionnaires are currently being used to determine physical activity [73–76]. A validated, self-administered questionnaire, the Pregnancy Physical Activity Questionnaire (PPAQ) has been used to assess the physical activity levels of pregnant women [74]. Categories in this questionnaire include: household/care-giving, occupational, sport/exercise, transportation and inactivity [77] and asks women to estimate the duration and frequency of time spent per activity during the current trimester of pregnancy. The reliability of the PPAQ for total physical activity was strong (r = 0.78), with the highest reliability for moderate intensity activity (r = 0.82). With regards to activity type, the highest reliability was found for occupational activity (r = 0.93), followed by household/care-giving (r = 0.86) and sports/exercise (r = 0.83). The validity of the PPAQ was determined against accelerometry (ActiGraph). The overall correlations between the PPAQ and average counts per minute were within the range of values observed for the published cut points (r = 0.27 for total activity), while validity coefficients for vigorous activity (r = 0.37) and sports/exercise (r = 0.48) were higher using average counts per minute. PPAQ provides an easy method of assessing physical activity patterns in women with uncomplicated pregnancies [77].

Both accelerometers [55, 78] and heart rate monitors [79] have been used to measure daily physical activity accurately. However, when these devices are used separately, they have disadvantages [80]. Heart rate is influenced by temperature, humidity, fatigue and emotional stress. [81]. Additional challenges are the loss of data from signal interruptions and delayed heart rate responses [82, 83]. Accelerometers on the other hand are not waterproof and cannot monitor activities in water [80]. Also, static physical activity, such as weight lifting, generates less body movement but requires energy expenditure, which can be problematic when accelerometers are used [84, 85].

To continually measure free-living physical activity, a combination of the abovementioned accelerometers and heart rate monitors are used and could provide more accurate activity profiles by overcoming individual sources of error [84, 86–89]. One such device that combines heart rate and accelerometry is the ActiHeart® (CamNTech, UK) [80], which was first used by Melzer et al. [35] to measure changes in resting and activity-related energy expenditure during pregnancy. The device is currently the only commercially available device that combines acceleration and heart rate, therefore increasing the practical applicability to improve energy estimates compared to traditional acceleration devices [90]. ActiHeart® is a 10 g, waterproof, self-contained logging device that allows activity to be measured synchronously with heart rate at between 15–60 s epochs [91]. The device is worn on the chest and consists of two electrodes that are connected by a short lead and clip onto two standard electrocardiograph (ECG) pads. Free-living data, as assessed by the ActiHeart® and calculated according to branched models, is essential to determine behavioural changes in activity patterns in pregnant women [35]. The ActiHeart® device has shown accurate estimates of energy expenditure versus indirect calorimetry over a wide range of activities (varying from sedentary behaviours to vigorous physical activity) in men and nonpregnant women, although it is not validated specifically for pregnant women [35]. Brage et al. [92] conclude that the ActiHeart® is a reliable and valid tool for the measurement of movement and heart rate in humans at rest and during walking and running. Overall, the ActiHeart® is reliable in measuring and categorising intensities of physical activity [80] in addition to increased monitor-wear compliance in adolescents [93] (Fig. 16.3).

The complexity of assessing physical activity in general, and in particular, during pregnancy, a demanding period characterised by changing physiology, hampers the determination of the optimal dose of recreational physical activity for pregnant women [3]. Because of the well-documented advantages of regular exercise in nonpregnant women, similar findings are expected during pregnancy. A lack of measuring instruments limits studies on the direct effect of physical activity levels on the growth of the foetus and maternal and foetal birth outcomes. The results are that health professionals have been very conservative in the volume (intensity x duration) and frequency of exercise and physical activity that are recommended to pregnant women. These guidelines have therefore impacted directly on habitual activity patterns during pregnancy.

Physical activity is a major determinant of lifelong health [121, 122] and has been associated with reduced morbidity and mortality [123–125] by serving as a primary preventive behaviour for several chronic health conditions including coronary heart disease [126–128], cancer [128], type 2 diabetes [129], [130], stroke [131], metabolic syndrome [132] and osteoporosis [133].

Maternal benefits of physical activity appear to be both physical and psychological in nature [10]. Physical benefits during pregnancy include shorter labour and a lower incidence of operative abdominal and vaginal deliveries and acute foetal distress [2, 128, 134–136]. Benefits for pregnant women also include improved cardiovascular function [2], reduced incidence of muscle cramps and lower limb oedema [137, 138], attenuation of gestational diabetes mellitus [139, 140] and gestational hypertension [24].

Physical activity does not only have physical benefits but also improves psychological health and provides wellbeing benefits [105, 141, 142]. An increased level of physical activity is known to have a protective effect against insomnia, stress, anxiety and depression [143–146], relieve job strain [147] and provide mood stability [66, 148] as well as increased perceived levels of energy during the day [149]. These benefits carry over to the postpartum period [149] and do not compromise infant breast milk acceptance of infant growth [150] .

Kalisiak and Spitznagle [151] reviewed clinically controlled trials that demonstrate that there is a moderate amount of evidence proving that exercise during pregnancy in healthy females has positive effects on both the mother and the foetus. While many studies conclude a positive relationship between physical activity and pregnancy outcome, the majority of the studies applied subjective questionnaires to determine the relationships. Therefore, accurate and objective methods to measure levels of physical activity are important when defining an appropriate relationship between physical activity and health outcomes for both the mother and foetus [90].

Recent meta-analyses of randomised control trails determining the effect of structured and supervised exercise during pregnancy report that pregnant women who exercised gained significantly less weight (− 1.13 kg) than women in the control group. The birth weight was however not significantly reduced in the exercise group compared to the control group [152].

Physical activity was discouraged until the early twentieth century on the basis of theoretical concerns about exercise-induced injury and adverse foetal and maternal outcomes [31, 144]. These concerns were based on the potentially detrimental effects of exercising on the mother and the foetus, secondary to increases in maternal body temperature, circulating stress hormones, caloric expenditure, decreased blood flow and biomechanical stress [153, 154] as seen in Fig. 16.5 [155].

Biological mechanisms that might contribute to reduced birth weight and length of gestation were theorised by [156]. They suggest that these effects are mediated by the sympathetic nervous system and may also be associated with the release of prostaglandins into the maternal circulation. Physical strain may lead to the release of catecholamines, which may increase maternal blood pressure and uterine contractility and decrease placental function [157].

Another concern of physical training during pregnancy is the subsequent teratogenic effect of hyperthermia in the first trimester [7, 155, 158]. However, this has not been shown to occur in studies of exercising women [8], because an increase in minute ventilation and skin blood flow augment heat dissipation and somewhat inhibit the potential hyperthermic effects of exercise [159]. Even so, exercising while pregnant should preferably take place in a well-ventilated and temperature-controlled environment [7].

The theoretical risk of foetal hypoxia is another concern for the exercising pregnant woman. It was once believed that the demands of exercising muscles divert blood flow from the uteroplacental unit [10]. However, compensatory changes with exercise, such as raised maternal haematocrit and oxygen extraction, appear to prevent the impairment of foetal oxygenation [135, 160]. Takito et al. [161] found that maintaining specific standing postures for a prolonged period could potentially reduce uteroplacental blood flow and lead to decreased foetal growth . Decreased visceral blood flow is suggested to cause potential adverse outcomes, such as congenital malformation, growth retardation, premature labour, brain damage, difficult labour, haemorrhage and maternal musculoskeletal injury [153].

Takito et al. [161] identified high total energy expenditure to potentially be associated with low birth weight, preterm birth and intrauterine growth restriction under the supposition that higher caloric expenditures could withhold energy from the foetus. The risk of maternal musculoskeletal injury due to changes in posture and centre of gravity or fetoplacental injury caused by blunt trauma or stress effects from sudden motions is also a concern [162].

Recommendations of physical activity during pregnancy before the twentieth century were overly conservative [162–171]. Recently, the guidelines have evolved as more reliable research has emerged [14]. The American College of Obstetricians and Gynaecologists (ACOG) found no scientific support that normal pregnant women should limit their exposure to physical activity based on the risks to the foetus and/or mother. However, some studies found that higher daily physical activity is inversely associated with foetal growth [172] and birth weight [173].

Campbell and Mottola [174] found that excessive physical exercise, at a frequency greater than 5 days a week, resulted in a low birth weight. However, their results also showed an equally harmful effect on foetal growth in the group of women who exercised less than two times per week. Magann et al. [175] supported the abovementioned results and found that less energy expenditure, at work and during leisure time, was associated with an increased risk of preterm birth and low birth weight (< 10th and < 3rd percentile).

The risk–benefit balance of physical activity during pregnancy needs to be assessed. During pregnancy, the risk of a sedentary lifestyle may be more detrimental than an active one [7], since a sedentary lifestyle includes loss of muscular and cardiovascular fitness, excessive weight gain, raised risk of gestational diabetes or pre-eclampsia, development of varicose veins and an increased risk of physical complaints such as dyspnoea, lower back pain and poor psychological adjustment [115, 139, 176]. According to Takito et al. [161], both excessive and insufficient physical activity impact negatively on pregnancy outcomes. Physical activity, done at an appropriate level for the physical condition of the woman, is beneficial to foetal growth , with the extremes being inactivity/sedentarism and a prolonged duration of vigorous intensities, which are potentially harmful to the supply of oxygen for adequate foetal growth [160]. However, women with complicated pregnancies have been discouraged from participating in exercise activities for fear of impacting the underlying disorder or maternal or foetal outcomes [8]. Some publications indicate that high levels of strenuous, high-intensity activity may result in preterm labour in susceptible individuals as well as babies with a low birth weight [177–179].

Absolute contraindications to exercise in pregnancy include haemodynamically significant heart disease, restrictive lung disease, incompetent lung disease, multiple gestation at risk for premature labour (≥ triplets), persistent second- or third-trimester bleeding, placenta praevia after 26 weeks’ gestation, ruptured membranes, preterm labour, pre-eclampsia, uncontrolled type-1 diabetes and thyroid disease or other serious systemic disorders like chronic bronchitis and uncontrolled seizures [8]. Relative contraindications to exercise include anaemia (defined by the World Health Organization as < 19 g/dL in pregnant women), unevaluated maternal cardiac arrhythmia, extreme morbid obesity and extreme underweight (BMI 8] (Table 16.1).

However, pregnant women should be advised that adverse pregnancy or neonatal outcomes are not increased for exercising pregnant women [7, 180–186], and maternal and infant health can even be enhanced [144, 180, 187–191]. Table 16.2 provides evidence regarding the effects of physical activity on foetal growth and birth outcomes.

Monitoring the growth of the foetus is a major purpose of antenatal care [201]. The overall term “foetal growth parameters” includes: head and abdominal circumference, femur length, ponderal index (weight in grams x100 divided by length in cubic centimetres), placental weight and expected birth weight [202]. While birth weight is a crude measurement of foetal growth, the measurement of head size and length at birth gives an insight into the timing of growth retardation during intrauterine life [203].