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Exercise in Pregnancy: How Safe Is It?

Arena, Bruno M.D.*; Maffulli, Nicola M.D., M.S., Ph.D., FRCS(Orth)

Section Editor(s): Maffulli, Nicola M.D.

Sports Medicine and Arthroscopy Review: March 2002 - Volume 10 - Issue 1 - p 15-22
The Female Athlete
Free

The role of exercise in pregnancy (EIP) is widely debated. Pregnancy produces marked changes of several physiologic variables of the mother. Regular EIP limits the increase in O2 requirement and produces an increase in the absolute anaerobic threshold, which persists for up to 36 weeks. Exercise in pregnancy reduces the incidence of muscle cramps, lower limb edema, fatigue, and shortness of breath. Increased knee cruciate ligament laxity does not normally produce any instability. Exercise in pregnancy reduces the increase in baseline maternal heart rate that occurs in pregnancy. Heart rate and stroke volume increase more markedly with exercise in pregnant women than in nonpregnant controls. Noradrenalin response to EIP is lower than in nonpregnant controls. Blood glucose levels decrease more promptly and to lower values with exercise in pregnant women than in nonpregnant controls. Glucose tolerance normally decreases in pregnancy, but increases during EIP. Regular EIP improves glucose tolerance in diabetic mothers and reduces total maternal weight gain and subcutaneous fat deposition toward the lower end of the normal range. Regular EIP produces higher plasma endorphin levels during labor and better pain tolerance than in sedentary controls. Women who undertake regular EIP have a lower incidence of 3rd- and 4th-degree vaginal tears. Children of exercising mothers have similar birth-weights as children of sedentary mothers, and mental performance at age 5 is higher. If there are no specific obstetric or medical contraindications, fit pregnant women can safely maintain the same level of fitness during pregnancy, although exercise schedules may have to be reduced.

From *Servizio Materno Infantile, Azienda Sanitaria Locale, Corso Vittorio Emanuele 163a, 29100 Piacenza, Italy; and the **Department of Trauma and Orthopedic Surgery, Keele University School of Medicine, North Staffordshire Hospital, Thornburrow Drive, Hartshill, Stoke on Trent, Staffordshire, ST4 7QB, England.

Address correspondence and reprint requests to: Nicola Maffulli, M.D., Department of Trauma and Orthopedic Surgery, Keele University School of Medicine, North Staffordshire Hospital, Thornburrow Drive, Hartshill, Stoke on Trent, Staffordshire, ST4 7QB, England. E-mail: n.maffulli@keele.ac.uk

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INTRODUCTION

Increasing numbers of women engage in sport to keep fit, often in the reproductive years. The decision to stop or curtail a program of regular exercise when a pregnancy is diagnosed should therefore be based on objective and reliable information. Often, women may be advised by friends or by their doctors to turn to a more sedentary lifestyle when pregnant, on the assumption that exercise may increase the risk of miscarriage or cause harm to the developing fetus. On other occasions, women themselves may make this decision.

Not all sports have the same impact on pregnancy, and intensity and duration of exercise sessions should be taken into account. Leisure exercise, such as moderate walking or biking or swimming, generally involves aerobic work and causes limited increases in maternal heart rate and ventilatory frequency. On the other hand, competitive sport may involve frequent maximal or near-maximal performances when the aerobic–anaerobic threshold may be reached. Maternal level of training and general fitness, particularly respiratory and cardiovascular reserves, set the safe level of exercise affordable by a pregnant woman.

In at-risk pregnancies, discontinuation of exercise may be necessary, and it is important that such cases are managed by expert medical personnel. In the absence of maternal cardiovascular or respiratory problems, moderate exercise makes the mother enjoy a sense of well being, is often associated with a shorter labor and a lower incidence of operative deliveries, and may help in the management of conditions such as diabetes and hypertension. The influence of regular maternal exercise on fetal development is controversial, because diversion of blood from the placenta to maternal muscles and other organs involved in the exercise is deemed by some authors to interfere with normal fetal growth. At present, a significant difference in birth weight between babies born to exercising mothers and sedentary controls has not been confirmed. Other studies evaluating mental and physical development of children born to exercising mothers also show encouraging data.

Finally, an important issue for its impact on pregnancy outcome is maternal physical activity in the workplace. Working mothers who spend many hours standing are at significant risk for premature labor in comparison to sedentary controls, while the difference in birth weight appears to be less significant. The quality of nutrition and the energy expenditure to care for other children at home also deserve careful consideration.

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PHYSIOLOGIC CHANGES IN PREGNANCY

The increased hormonal production from the feto-placental unit during pregnancy produces significant changes in several maternal parameters. Oxygen consumption (Vo2) increases, reaching its maximum at approximately 32 weeks. Ventilation, tidal volume, heart rate, and stroke volume also increase gradually. Carbon dioxide output (Vco2) follows the same trend as Vo2, and peripheral resistance decreases while circulating plasma volume increases, producing a drop in the hematocrit. Body temperature rises by about O.5°C. Forced vital capacity remains relatively constant, whereas expiratory reserve decreases. 1,2

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Exercise and Maternal Respiratory System

Carbon dioxide output at peak exercise is lower during pregnancy than during the postpartum period, 3 and submaximal exercise produces higher ventilatory frequency, tidal volume, and lactate levels than in nonpregnant controls. This response decreases gradually after delivery, but it takes several weeks to return to the normal, nonpregnant state. Fit women who continue to exercise regularly during pregnancy can engage in heavier physical activities than sedentary pregnant controls, without causing fetal hypoxia: the immediate postexercise recovery period can be more risky for the fetus, probably due to the fall in stroke volume. 4 Physical activity at moderately high altitudes can affect physical performance during pregnancy: a passive trip by aerial cableway in the third trimester to an altitude of 2200 m, which is equivalent to a drop in barometric pressure to 583 mm Hg and a drop in atmospheric Po2 of 18 mm Hg, does not produce any change in maternal Po2, respiratory rate, heart rate, systolic and diastolic blood pressure at rest 5; however, the maximum oxygen consumption (Vo2max) of pregnant women at low altitudes can be limited, although performance of submaximal exercise may be unaffected. 4 The type of sport practiced by a pregnant woman is important, because not all disciplines have the same effect on the maternal respiratory system. For example, immersion in water to the level of the xiphoid in pregnancy produces a decrease of maximal voluntary ventilation, compared with rest on land or exercise in water. 6 In addition, perceived maximal exertion is reached at a lower percentage of Vo2max during swimming, compared to cycling. Peak co2 output, peak ventilation, and lactic acid production are also lower during swimming than during cycling. 7

The anaerobic threshold (AT) normally decreases with advancing gestation in proportion to increasing maternal body weight. The maternal heart rate at the AT also declines, but without close relation to maternal body weight. Maternal heart rate increases with more difficulty during exercise with advancing gestational age, and this has to be taken into account when the maximal heart rate is evaluated as an index of exercise intensity. 8 On the other hand, in recreational athletes who continue to exercise during pregnancy, albeit at a reduced intensity, there is an increase in the absolute anaerobic threshold, which persists up to 36 to 44 weeks postpartum. Weight and maximal heart rate remain unchanged. 9

The amount of oxygen required to complete a three-step graded workload treadmill exercise test decreases significantly by 6% to 15% starting from the first weeks of pregnancy. In women who continue a moderate to high intensity exercise regime, oxygen requirements remain the same or are lower than before conception for the rest of the pregnancy and early postpartum. On the contrary, women who discontinue regular exercise experience a progressive increase in oxygen requirements by 2% per month, peaking at 37 weeks. The additional oxygen requirement caused by weight gain must be taken into account when net physical efficiency is evaluated. 10 In fact, physical efficiency improves during normal pregnancy compared with the nonpregnant state, with an increase in the Vo2max and a lower production of lactate when the AT is exceeded. 11 A significant increase in Vo2 uptake during exercise is observed in late pregnancy, compared to the postpartum period, if measurements are taken at rest, during a steady-state exercise on a bicycle ergometer, and for 10 minutes during recovery. The oxygen debt produced by the exercise is also higher compared to 12 weeks postpartum. 12 Maximal transdiaphragmatic pressure remains unchanged when healthy women perform progressive cycle exercise tests at 33 weeks and 12 weeks postpartum, and the higher tidal volume produces an increase in minute ventilation. 13

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Exercise and the Maternal Musculoskeletal System

Women who exercise during pregnancy report less frequent legs edema, muscle cramps, fatigue, and shortness of breath than sedentary pregnant controls. 14 Abdominal muscle function is affected by the structural adaptations that occur during pregnancy, particularly the increasing size of the uterus, and the ability to stabilize the pelvis against resistance decreases until approximately 8 weeks postpartum. 15 In addition, there is generalized ligament laxity, a result of increasing levels of relaxin. This is a peptide hormone of the insulin-like growth factor family, produced since the first weeks of pregnancy by the corpus luteum and the placenta. Laxity of the cruciate ligaments of the knee is common, but exercise programs using minimal to moderate weight bearing do not produce abnormal knee laxity. 16 Severe low back pain that interferes with daily life activities develops in approximately 10% of pregnant women, and the ability to perform sit-ups decreases significantly because of the inefficiency of the abdominal muscles. 17

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Exercise and Maternal Cardiovascular System

Exercise produces an increase in cardiac output. The blood flow to muscles, myocardium, and skin increases, whereas sympathetic vasoconstriction reduces the flow to organs and tissues not directly involved in the exercise. The response to exercise, although influenced by physiologic conditions such as menstrual cycle and pregnancy, does not differ significantly from the one occurring in men. 18 The cardiac response to exercise in pregnancy (increased heart rate and stroke volume), which is greater than in nonpregnant controls, decreases by 2 months postpartum, but a longer period of time is required to return to nonpregnant conditions. 19 Activation of the sympathetic nervous system, either at rest or during strenuous exercise, is blunted in pregnancy, and the circulatory system is affected by the increased activity of the renin–angiotensin system. 20 In particular, plasma noradrenalin increases less significantly in pregnancy during standing and isometric exercise than in nonpregnant controls. Adrenalin response is less affected by pregnancy, with a linear correlation between heart rate and urinary excretion of adrenalin, both at rest and during exercise. 21

Changes in submaximal exercise Vo2 during pregnancy are dependent on the type of exercise. At the same workload, Vo2 increases during weight-bearing exercise but does not differ from postpartum values during weight-supported exercise. Exercise arterial-venous oxygen difference is lower during pregnancy than in the nonpregnant state, probably because the higher cardiac output is distributed to vascular districts not directly involved in the exercise. Perfusion of muscles would not increase significantly during exercise, and the increased cardiac output can therefore cover the requirements of blood flow to the uterus. 22

Aerobic dancing at an intensity of approximately 65% of expected maximal heart rate in the third trimester produces a significant increase in blood pressure and an increase in heart rate that may continue for up to 20 minutes. Systolic peak velocity and flow volume in the femoral arteries increase significantly, and the pulsatility index decreases. On the fetal side, the systolic peak velocity and the pulsatility index of the umbilical artery do not change significantly, but the fetal heart rate can increase in relation to maternal heart rate. 23 When exercise in water is evaluated, immersion produces a decrease in vasopressin production in healthy pregnant women, which increases instead after exercise. Plasma renin activity decreases both during and after exercise. Atrial natriuretic peptide concentration increases after exercise in water and affects the volume of urine produced. 24 Immersion in water for 20 minutes at 30°C at 15, 25, and 35 weeks of pregnancy and 8 to 10 weeks postpartum produced a decrease in the resting heart rate by approximately 8 beats per minute (bpm). Twenty minutes of bicycle ergometry to 60% of Vo2max in water determined an increase in the heart rate lower than the one produced by the same exercise on land, in all stages of pregnancy. 25 The positive effects of exercise in water have been confirmed in another study, in which immersion for 20 minutes at 30°C in pregnancy followed by exercise to 60% of Vo2max produced substantial diuresis and natriuresis without changes in osmolarity or serum sodium. Exercise in water during pregnancy can therefore be potentially beneficial to treat edema, without producing a decrease in plasma volume. 26

Specific electrocardiographic changes occur in pregnancy: T-wave inversion in V2 is present in early and late pregnancy more frequently, while Q-waves in II, III, and aVF are less common than in nonpregnant subjects. With bicycle exercise, the time to onset of maximum ST depression is significantly shorter during pregnancy. 27 In another study, a depression of ST segment was found in 12% of pregnant women undergoing a strenuous bicycle exercise who did not show any signs of ischemia. 28 Knowledge of this helps in the correct evaluation of the electrocardiogram in pregnancy.

The systolic–diastolic (S/D) ratio remains basically unchanged, when measured in the uterine artery before and within 3 minutes of a submaximal stationary bicycle exercise in healthy women with singleton pregnancies between 16 and 28 weeks. This suggests that submaximal exercise does not compromise uterine artery blood flow in healthy women with normal pregnancies. 29 Also, peak velocities and mean blood flow velocities increase with no variation in the vessel diameter in the femoral and carotid arteries and femoral vein of healthy pregnant women, performing sitting bicycle ergometry with a workload of 100 W for 3 minutes. The systolic and end-diastolic velocities in the femoral artery also increased, with a reversal of the post-systolic flow, which normally shows negative velocity at rest. 30 An improvement in maternal circulation with exercise has been confirmed by other studies: a bicycle stress test in the third trimester, by healthy pregnant women in a semi-supine position at 75 W for 3 minutes, shows a significant decrease of the resistant index (RI) of the maternal femoral artery (from 93% to 69%), a rise in maximum systolic velocity (from 73 to 194 cm/sec), and a rise in maximum diastolic velocity (from 5 to 61 cm/sec). In the maternal carotid artery and uteroplacental vessels, these parameters remain basically unchanged. Fetal cardiotocograph results remain within normal limits in all cases. These studies confirm that, provided placental function is normal, uteroplacental and fetoplacental circulations are not affected negatively by moderate physical exercise. 31

When the exercise intensity increases, there can be negative effects on uterine circulation. Submaximal maternal exercise at approximately 75% of Vo2max during the third trimester can induce a gradual increase of the S/D ratio of the uterine artery, with a maximum at one minute of recovery. No change has been found in the S/D ratio of the umbilical artery, but the fetal heart rate can increase markedly. 32

Maternal blood pressure response to exercise is not affected by pregnancy and is inversely related to the individual capacity to perform isometric exercise. Isometric exercise (hand-grip test) at 28 weeks predicts in which women pregnancy-induced hypertension and preeclampsia will develop. The test was positive when the systolic blood pressure increased by 15 mm Hg or more during isometric exercise or decreased by 14 mm Hg or more immediately after isometric exercise. 33,34

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Exercise and the Feto-Placental Unit

Exercise confined to early pregnancy increases the parenchymal component of the placenta, total vascular volume, and surface area. Exercise carried out throughout pregnancy increases these and other histomorphometric variables associated with the rate of placental perfusion and transfer, with significant changes confined only to villi with a diameter of more than 80 μm. 35 Placental blood flow is lower in pregnant women with hypertension, diabetes, and cholestasia than in healthy controls, but it increases within a few minutes of submaximal exercise. Conversely, in normal pregnancies, a short submaximal exercise has little effect on placental blood flow. 36 Even though it has no significant effects on uterine artery perfusion, submaximal exercise can be associated with an increase in the fetal internal carotid artery mean velocity and a decrease in the cerebral resistance index, consistent with a slight fetal cerebral vasodilatation. 37 The S:D ratio of the umbilical artery close to the placental insertion site does not change significantly in the third trimester, before and after a submaximal exercise bout at 85% of predicted maximum heart rate on a bicycle ergometer. 27 A lack of effect on the umbilical artery S:D ratio has been found also in growth-retarded fetuses with placental dysfunctions, following graded walking on a treadmill with an upper limit of maternal heart rate of 150 bpm. 38 Elevated erythropoietin levels in the fetal compartment (cord blood and amniotic fluid) likely reflect fetal hypoxemia, but erythropoietin levels obtained at the time of membrane rupture do not differ in women who exercise regularly until the onset of labor and nonexercising controls. This confirms the absence of detrimental effects of regular maternal exercise on fetal oxygenation. 39

Moderate to heavy maternal exercise, such as a bicycle exercise test at 82% of maximal increase in maternal heart rate between 29 and 32 weeks, can produce an increase of the fetal heart rate (FHR) for 30 minutes, and a reduction in the FHR variability for 20 minutes compared to pre-exercise parameters. Fetal body movements are also affected, with a reduction after exercise that may last for as long as five minutes. 40 A brief submaximal maternal exercise up to approximately 70% of maximal aerobic power (maternal heart rate not more than 148 bpm) can occasionally produce decreases in fetal heart rate. Maximal exertion generally is followed by fetal bradycardia, probably because of inadequate fetal gas exchange. 41 In most cases, FHR tends to increase during mild, moderate, and strenuous maternal exercise; bradycardia occurs only sporadically. Variations in FHR during exercise are not directly correlated with gestational age, exercise intensity, and maternal levels of circulating catecholamines. 2 When light maternal exercise is performed at an altitude of 2500 m, no significant FHR changes are recorded in normal pregnancies. The same cannot be recommended for women with high-risk pregnancies. 42

Fetal heart rate, cardiac size, and fetal ventricular function do not change significantly 5 minutes after a short-term moderate maternal exercise between 18 and 36 weeks of pregnancy (bicycle ergometer load test of 5 W), in spite of a median maternal heart rate acceleration to 68% of the expected maximum heart rate 43; and, with maternal heart rate reaching, during a 15-minute graded treadmill exercise, 168 bpm (84% training intensity), FHR increased significantly only in 4 of 10 women. 44 In this study, the baseline FHR remained in the normal range (12–160 bpm) in women whom training intensity was below 70% of expected maximal maternal heart rate.

Fetal breathing movements respond differently according to the type of exercise performed by the mother: they increase after dynamic work (bicycle exercise), remain unchanged after passive or isometric muscle contraction, and decrease after maternal hyperventilation and hyperoxygenation. Fetal breathing movements appear to respond to environmental changes more promptly than fetal heart rate, and co2 stimulates breathing movements also in utero. 45 Unlike breathing movements, which tend to decrease after 20 minutes of aerobic dance carried out at 35 weeks, fetal shoulder movements or kick responses show no significant variation. 46 During immersion and exercise in water at 60% of Vo2max in pregnancy, unlike what occurs with exercise on land, fetal heart rate does not vary significantly, and no uterine activity is recorded either at 25 or 35 weeks. Plasma volume expansion taking place with immersion may contribute to preserve the normal FHR. 47

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Exercise and Feto-Maternal Metabolic Changes

During exercise, there is an increase in tissue uptake of glucose and insulin sensitivity. Serum adrenalin, noradrenalin, growth hormone, cortisol, and glucagon increase with a breakdown of hepatic and muscular glycogen. Failure of this mechanism to function can result in hypoglycemia, 48 and animal studies have shown that recurrent acute exercise during pregnancy can have detrimental effects on fetal development only if dietary glucose is severely restricted. 49 Blood glucose levels of pregnant women decrease at a faster rate and to significantly lower values after 1 hour of moderate intensity exercise (55% VO2max) in the third trimester, compared to nonpregnant controls. Insulin levels also decrease, and lactate levels are maintained at a lower level 15 minutes after exercise. 50 Strenuous exercise produces an increase in mean values of [H(+)], Pco2 and total protein and a decrease in bicarbonate concentration in physically active pregnant women at a gestational age of 33 weeks, and in age-matched nonpregnant controls. At rest and during postexercise recovery, pregnant women show significantly lower mean values of Pco2 and total protein. 51

Glucose tolerance decreases during pregnancy. This is a consequence of increased peripheral insulin resistance, and physical exercise can improve both glucose tolerance and the insulin response to a glucose load. 52

Pregnancy decreases the net cost of weight-dependent and weight-independent standard exercises when expressed per kilogram of body weight 53 and determines a progressive increase in 24-hour energy expenditure proportional to body weight and significant at 36 weeks of gestation. 54 It also reverses the hyperglycemic response to intensive exercise that occurs in nonpregnant women in proportion to the intensity of the exercise. At 8 weeks, blood glucose levels increase only when exercise intensity exceeds 80% of maximum. At 15 weeks, this trend is even more marked, but by the 23rd week, exercise determines a decrease in blood glucose level. The change in glucose response occurring since the first trimester of pregnancy may be caused by decreased hepatic glucose release associated with increased peripheral glucose oxidation, which increases in the last part of pregnancy for the requirements of placenta and fetus. 55

Moderate exercise by insulin-dependent pregnant women to improve glucose tolerance is useful and does not cause any fetal or maternal complications, 56 whereas uptake of glucose by the fetus may be compromised after exercise only as a result of excessive glucose uptake by maternal skeletal muscles. 57 Twenty-four-hour whole body calorimetry before pregnancy and at 6, 12, 18, 30, and 36 weeks of gestation shows a significant decrease of basal metabolic rate up to 24 weeks of gestation. At 36 weeks, basal metabolic rate increases by +8.6% to +35.4% relative to the baseline before pregnancy. Even if caloric intake is kept constant, body weight and fat increase progressively until delivery, but women who continue to exercise regularly accumulate less fat in the first and second trimesters and after delivery than nonexercising controls. 58,59 Weight retention one year after delivery is higher in women who increase their caloric intake during and after pregnancy. There is a negative correlation between the degree of physical activity in the postpartum period and weight retention. 60 Prenatal exercise is one of the most important parameters, in addition to prepregnancy weight, gestational weight gain, and parity, in influencing postpartum weight gain. 61

Plasma corticotropin, cortisol, and glucose levels increase less markedly in response to a stressor event, such as a treadmill exercise set at 90% of Vo2max, in lactating women between 7 and 18 weeks postpartum, compared with nonlactating controls. This suggests that stress-responsive neurohormonal systems are less responsive in women who are breast feeding. 62 Exercise appears also to affect prostaglandin metabolism in pregnancy; pregnancy, in fact, is characterized by a 3.6- to 4.3-fold increase in urinary prostacyclin excretion with no significant change in thromboxane output, but after 30 minutes of submaximal exercise, prostacyclin and thromboxane excretion increase. 63

An important issue regarding safety of exercise in pregnancy is the increase in body temperature. In a rat model, exercise in hot conditions during the first trimester of pregnancy is associated with a significant increase of fetal anomalies, particularly neural tube defects. 64 This has not been confirmed in humans. Later in pregnancy, high-intensity exercise, such as upright cycling for 15 minutes at 87.5 W (maternal heart rate 156 bpm), produces higher maternal temperature and fetal heart rate changes than lower-intensity exercise of longer duration, such as cycling for 30 minutes at 62.5 W (maternal heart rate 142 bpm). In either case, the temperature does not rise beyond 38°C, nor does it produce adverse fetal heart rate changes. 65

Isometric exercise, such as a hand-grip test in the third trimester of pregnancy, causes a significant increase in plasma corticotropin and immunoreactive β-endorphin concentrations in normal and hypertensive pregnancies. 66 Women who exercise throughout pregnancy show higher plasma beta-endorphin levels than nonexercising controls throughout labor and a reduced pain perception. Cortisol, growth hormone, and prolactin levels are also lower during labor. 67 Mean basal concentration of corticotropin is higher in women with pregnancy-induced hypertension than in normal controls, whereas β-endorphin concentration does not differ significantly. In response to a hand-grip test, concentrations of both substances increase significantly in both groups. The increased basal concentration of corticotropin in women with pregnancy-induced hypertension has been linked to increased circulating levels of corticotropin-releasing hormone. 66 The secretion of beta-endorphins increases in response to exercise both in nonpregnant and pregnant women, but the response lasts longer in pregnant women. 68 Plasma beta-endorphin concentration is higher in women who undergo aerobic training on a bicycle ergometer during pregnancy than in sedentary controls. This difference is present also during labor. Pain perception is also reduced in patients who exercise, compared with sedentary controls. Cortisol, human growth hormone, and prolactin levels are lower during labor in exercise-conditioned patients. 67

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Exercise, Labor, and Delivery

Endurance training (cross-country skiing, running, speed-skating, or orienteering) had no harmful effects on labor or delivery in a group of Finnish élite athletes. 69 Women who practice sport regularly have a smaller incidence of third- and fourth-degree vaginal tears than sedentary controls. 70

As to the influence of different sports on early miscarriage, in a prospective study on exercise performance before and during pregnancy in 47 recreational runners, 40 aerobic dancers, and 28 physically active fit controls, spontaneous abortion occurred on the whole in 19% of pregnancies. The rate of spontaneous abortion was 17% in runners, 18% in aerobic dancers, and 25% in the control group. One congenital abnormality was detected at term in each group. 70 Contractions appeared to depend more on the type of exercise than on the level of exertion. At equivalent levels of aerobic exercise, bicycle ergometry produces the onset of uterine contractions in 50% of sessions, treadmill running in 40%, and rowing ergometry in 0%. 71 Hypertensive pregnant women undergoing a short-term submaximal bicycle ergometer test are more prone to the onset of uterine contractions than healthy women or women with other medical problems, such as diabetes or intrahepatic cholestasis of pregnancy. 72

Treadmill exercise for periods of 10 minutes in the second and third trimesters and after delivery increases uterine activity in 10 of 14 women during exercise or recovery. Fetal heart rate increases with exercise independently of uterine contractions and plasma noradrenalin levels. 73 Women who practice aerobic exercise during pregnancy tend to have a shorter second stage of labor and fewer obstetric complications than sedentary controls, 74 and birth weight and neonatal Apgar scores remain unchanged.

The absence of negative effects of moderate exercise on uterine activity has been confirmed in a study of 102 healthy pregnant women between 25 and 38 weeks of pregnancy, who showed no statistically significant difference in the number of contractions 1 hour before and 1 hour after gymnastics. 75 Women who perform regular leisure-time physical activity during the first half of pregnancy show a reduced risk for preeclampsia (relative risk [RR] 0.67) and gestational hypertension (RR 0.75), the more so the longer the average time spent in the leisure-time physical activity. 76

No association has been found between the frequency of uterine contractions and smoking, use of alcohol and coffee, or prolonged standing or lifting in pregnant women at low risk after 30 weeks of pregnancy. Uterine contractions appear to increase to a small degree only with climbing stairs and walking, 77 and upper extremity exercise does not produce uterine contractions, which are instead stimulated by lower extremity exercise. The former appears, therefore, advisable in diabetic pregnant women, when exercise is used in addition to insulin therapy for a better control of hyperglycemia. 78 Maternal exercise during labor, at an intensity of approximately 60% of Vo2max, decreases maternal pain perception. 79

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Physical Activity in the Workplace and Pregnancy Outcome

Women employed in jobs that require intensive physical activity are at lower risk for pregnancy-induced hypertension than are women employed in jobs entailing low levels of physical activity but standing for more than 8 hours per day. Cleaning house for more than 7 hours per week and looking after young children for more than 50 hours per week is associated, on the contrary, with a decreased risk. 80

Long working hours and fatigue seem to predispose pregnant women to premature delivery, whereas lifting heavy weights and shift work affect fetal growth and duration of pregnancy. 81 A study of 50 French factories has shown a significant relationship between periods of rest because sick leave prescribed especially for fatigue, without any specific pathologic cause, and a lower preterm birth rate. 82 Another study has shown that mothers involved in heavy labor have a mean pregnancy weight gain of 3.3 ± 2.4 kg, independent of the weight of their offspring, compared to a weight gain of 5.9 ± 3.3 kg for the less active mothers. 83

Duration of rest in women involved in hard work during pregnancy has a strong influence on birth weight and birth length in newborn girls, but a lesser influence on newborn boys. There is a 7.5-fold decrease in the rate of low birth weight in girls when the duration of maternal rest is longer than 21 days. In boys, the low birth weight rate does not change. 84

Increased observed–expected (O:E) ratios for late abortion have been found in operating room nurses, radiology technicians, and agriculture and horticulture employees. The latter also show a significantly raised O:E ratio for stillbirths. The O:E ratio for stillbirth can approach 2:1, and the abortion ratio can increase by approximately 20% in women exposed to solvents. 85 In another study on possible associations between structural fetal malformations and occupational exposure, in 1475 mothers of malformed babies and an equal number of mothers of healthy babies, a significant relationship has been found between physical load and growth retardation. Mothers employed in jobs that entail occasional high physical work loads show more frequent pregnancy-induced hypertension, and women who have to stand long hours because of their work show a higher risk for threatened abortion. 86

Many studies have evaluated the effect of stressors, such as prolonged standing, physical exertion, and long working weeks, on pregnancy. They suggest an influence on the duration of pregnancy (risk for premature delivery) rather than on birth weight. 87 In a study of women who had delivered babies weighing 2500 g or less and controls with normal-sized babies, no association was found between the level of activity at work of the mother and delivery of a baby with low birth weight. 88

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Exercise and the Neonate

Women performing more than four sessions of vigorous exercise weekly at 25 weeks of pregnancy tend to deliver babies whose mean birth weight is 315 g lower than the mean birth weight of neonates born to nonexercising controls. Participation in aerobic exercise during pregnancy does not adversely affect birth weight or other maternal and fetal parameters and may render pregnancy generally more comfortable. 89

A study of 2743 pregnant women divided into five groups on the basis of daily kilo energy expenditure has shown that women in the medium energy expenditure group give birth to babies of higher weight than those in the other groups, but mean birth weight within each group remains within the normal range. 90

Average birth weight and length of gestation do not differ significantly between women who continue to exercise during pregnancy and sedentary controls. 91 In physically trained pregnant women, maternal fitness appears to have no significant effect on the state of the newborn at birth, whereas the short labor duration in trained mothers reflects their higher levels of activity and fitness in all stages of labor. 92 When aerobic fitness is maintained during pregnancy, there is no increase in neonatal morbidity or obstetric complications, and mothers tend to experience a significant level of well-being. 93

In a recent study comparing the offspring of 20 women who exercised during pregnancy and those of 20 controls, head circumference and fetal length at birth were similar, but babies of exercising women weighed less and had less fat. At 5 years of age, head circumference and height were similar, but the children of the women who exercised weighed less and had a lower sum of five-site skinfolds. Motor, integrative, and academic readiness skills were similar, but performance on the Wechsler scale and tests of oral language skill were significantly higher in children of exercising women. 94

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Psychological Aspects of Exercise in Pregnancy

Adolescent pregnant women participating in a 6-week aerobic exercise program show a significant decrease in depressive symptoms and an increase in total self-esteem, while those in the control sedentary group report a significant increase in physical discomfort associated with pregnancy. Aerobic exercise programs during pregnancy have been shown to reduce depressive symptoms and increase total self-esteem in adolescents, compared to a nonexercising control group, and can represent an important aspect of antenatal care in adolescents. 95

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CONCLUSIONS

The American College of Obstetricians and Gynecologists guidelines on physical exercise in pregnancy 96 stress that, if there are no specific obstetric or medical contraindications, fit women can safely maintain the same level of fitness during pregnancy, although exercise schedules may have to be reduced. In healthy women, exercise during pregnancy at a level that does not produce exhaustion does not cause any significant risk for the mother or baby. Caution must be recommended for mothers with preexisting medical conditions or complications developing during pregnancy.

There are, at present, no conclusive data regarding the level of exercise that can cause a reduction of fetal weight at birth or, conversely, can definitely improve pregnancy outcome. Therefore, a program of physical exercise in pregnancy will have to be adjusted individually.

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Section Description

Nicola Maffulli, M.D., Guest Editor

Keywords:

Pregnancy; Exercise; Weight gain; Adverse effects; Beneficial effects

© 2002 Lippincott Williams & Wilkins, Inc.