In 2002, the American College of Obstetricians and Gynecologists published exercise guidelines for pregnancy, which suggested that in the absence of medical or obstetric complications, 30 minutes or more of moderate exercise a day on most, if not all, days of the week is recommended for pregnant women. However, these guidelines did not define ‘moderate intensity’ or the specific amount of weekly caloric expenditure from physical activity required. Recent research has determined that increasing physical activity energy expenditure to a minimum of 16 metabolic equivalent task (MET) hours per week, or preferably 28 MET hours per week, and increasing exercise intensity to ≥60% of heart rate reserve during pregnancy, reduces the risk of gestational diabetes mellitus and perhaps hypertensive disorders of pregnancy (i.e. gestational hypertension and pre-eclampsia) compared with less vigorous exercise. To achieve the target expenditure of 28 MET hours per week, one could walk at 3.2km per hour for 11.2 hours per week (2.5 METs, light intensity), or preferably exercise on a stationary bicycle for 4.7 hours per week (~6–7 METs, vigorous intensity). The more vigorous the exercise, the less total time of exercise is required per week, resulting in ≥60% reduction in total exercise time compared with light intensity exercise. Light muscle strengthening performed over the second and third trimester of pregnancy has minimal effects on a newborn infant’s body size and overall health. On the basis of this and other information, updated recommendations for exercise in pregnancy are suggested.

Artifact in the recording of fetal heart rates during material exercise.

Exercise guidelines in pregnancy: new perspectives.

Doppler monitoring of fetal heart rates during maternal exertion has suggested that fetal bradycardia occurs frequently during vigorous exercise, causing concern for fetal safety. Doppler determination of fetal heart rate during vigorous maternal effort is difficult. To avoid motion artifact, we observed fetal heart rate using two-dimensional ultrasound and determined the incidence of fetal bradycardia in 45 pregnant women (age, 29.0 +/- 3.7 years [mean +/- SD]; gestational age, 25.2 +/- 3.0 weeks) during 85 submaximal and 79 maximal cycle ergometer tests. Average fetal heart rate did not change during exercise. A single episode of fetal bradycardia (heart rate less than 110 beats per minute for greater than or equal to 10 s) occurred during submaximal exertion during a maternal vasovagal episode. Sixteen episodes of fetal bradycardia were noted within three minutes after cessation of exercise, 15 of which followed maximal maternal effort. We conclude that brief submaximal maternal exercise up to approximately 70% of maximal aerobic power (maternal heart rate less than or equal to 148 beats per minute) does not affect fetal heart rate. In contrast to submaximal maternal exertion, maximal exertion is commonly followed by fetal bradycardia. This may indicate inadequate fetal gas exchange.

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Fetal Heart Rate Response to Maternal Exertion

Alterations in maternal physiology during pregnancy affect the physiological response to aerobic exercise. Maternal resting oxygen consumption (VO2) and cardiac output (Q) increase during pregnancy. Heart rate (HR) becomes progressively elevated throughout gestation, whereas stroke volume (SV) increases until the third trimester and then declines until term, probably because of diminished venous return. Plasma volume increases earlier and to a greater magnitude than red cell volume, resulting in the ‘haemodilutional anaemia’ of pregnancy and a decline in the oxygen-carrying capacity. Ventilation is greater during pregnancy because of elevated tidal volume and unchanged rate of breathing.

Aerobic exercise during pregnancy. Special considerations.

SummaryMaternal exercise resulted in an increase in fetal heart rate and variability in normal fetuses. In small fetuses there was a variable response, with four of 20 fetuses having sharp decelerations in heart rate with maternal exercise.A rise in maternal core temperature resulted, in all cases, in a rise in maternal heart rate. In normal fetuses the fetal heart rate also rose by on average 12·2 beats per min per degree Celsius. In small for dates fetuses there was no significant change in mean fetal heart rate, with three of 11 fetuses having decelerations in heart rate with increased maternal temperature.Circadian variation seen in fetal heart rates was similar to that occurring in the mother (r = 0·49, P < 0·05). There was no demonstrable circadian variation in neonatal heart rate (r= -0.24). This suggests that the fetal heart rate variation is extrinsic in origin.

Artifact in the recording of fetal heart rates during material exercise.

Citations

Exercise guidelines in pregnancy: new perspectives.

Fetal Heart Rate Response to Maternal Exertion

Aerobic exercise during pregnancy. Special considerations.

Fetal heart rate responses to maternal exercise, increased maternal temperature and maternal circadian variation

The role of exercise in pregnant women with diabetes mellitus.