The biological adaptations to pregnancy strain the body beyond any other physiological event in a healthy woman’s experience, and requires significant cardiovascular, metabolic, hormonal, respiratory and musculoskeletal adjustments (Sternfeld, 1997). In addition, psychological responses to pregnancy (e.g., mood changes) and during postpartum (e.g., depression) have been well documented (Koltyn, 1997). Recent reviews and studies indicate that exercise not only has a positive effect on psychological outcomes during pregnancy, but moderate exercise can also have beneficial influence on ventilatory demand, perceived breathing effort in the late gestation period (Ohtake & Wolfe, 1998) and can contribute to significantly heavier birth weight babies (Pivarnik, 1998).

Physiological Concerns
The following are some of the physiological concerns regarding exercise during pregnancy:

Hyperthermia: Maternal temperature appears to be the primary regulator or thermostat of fetal temperature. If maternal temperature is increased excessively, it could threaten the health of the fetus. Research shows that when the level of exercise is self-paced, exercise during pregnancy results in only relatively moderate changes in maternal temperature (Sternfeld, 1997). Hence, the thermoregulatory processes during pregnancy seem to protect against hyperthermia when exercising at moderate levels (Canadian Academy of Sports Medicine, 1998).

Uterine Blood Flow: Uterine blood flow is part of the internal organs circulation system. Blood flow during moderate exercise in a non-pregnant state is redistributed away from the internal organs to the working muscles, up to 50% less than at resting values, and a further 30% reduction during prolonged high intensity exercise (Canadian Academy of Sports Medicine, 1998). The concern about exercise during pregnancy is a decrease in nutrient delivery to the fetus which may result in low birth weight, fetal hypoxia, and fetal growth retardation. Research shows that oxygen delivery to the fetus and fetal oxygen consumption are maintained at relatively constant levels during exercise due to the following mechanisms: a) increased oxygen-carrying capacity of the blood; b) increased oxygen extraction as blood flow decreases; and c) redistribution of the blood flow which benefits the placenta over the uterus. A possible negative effect on birth weight is associated only with high frequency (5 - 7 days/week) exercising. Moderate frequency and intensity exercise are found to have a positive effect on the weight of healthy babies (Pivarnik, 1998).

Uterine Contractions: Norepinephrine is released during exercise in both pregnant and non-pregnant states. Theoretically, norepinephrine is an uterine stimulus which could trigger uterine contractions and result in premature labour. Research evidence to date suggests that the fetus is protected from this stimulatory effect (Sternfeld, 1997). Exercise or other physical activity does not increase the risk of preterm labour, nor increase the occurrence of premature rupture of membranes in a healthy pregnancy (Canadian Academy of Sports Medicine, 1998).

Exercise prescription for pregnancy
The Canadian Academy of Sport Medicine recently published the Position Statement on Exercise and Pregnancy (Canadian Academy of Sports Medicine, 1998) that reviews the risks and benefits, and also gives specific recommendations on exercise prescription for this segment of the population. In addition, the Canadian Society for Exercise Physiology (CSEP) developed the Physical Activity Readiness Medical Examination for Pregnancy (PARmed-X for Pregnancy). Physicians can use this form for screening the health of pregnant patients prior to their participation in exercise programs and for continual medical monitoring.

Tips & Recommendations
The following guidelines are endorsed by the Canadian Academy of Sport Medicine (1998). See the Position Statement for a complete listing of the recommendations. Please note that these recommendations are largely based on studies of healthy, nonsmoking, Caucasian, previously fit women with low risk pregnancies.

• Regular exercisers prior to pregnancy may maintain their program during the first trimester and should use the PARmed-X for pregnancy guidelines throughout the pregnancy.
• Women who were not regular exercisers prior to their pregnancies should not begin to exercise until the second trimester.
• The frequency of exercise sessions per week is dependent upon the duration and intensity of the sessions. However, it is recommended to begin with a 3 times/week frequency and progress to a maximum of 4-5 times/week.
• Since the safe upper limit for an exercising target heart rate in pregnancy is controversial, the talk test (an intensity level where the exerciser can easily carry on a verbal conversation) and the Borg’s 15-point Rating of Perceived Exertion (RPE) scale (i.e., a target range of 12-14 points - moderate to somewhat hard intensity) are recommended to assess exercise intensity.
• Cardiovascular exercise should begin with 15 minutes in duration and slowly be increased by 2 minutes/week until a maximum of 30 minutes at the target heart rate.
• Non-weight bearing activities (e.g., swimming and cycling) are ideal for women who were inactive prior to their pregnancies.
• Holding your breath during resistance training and exercises in the supine position after the fourth month of pregnancy should be avoided.

Conclusion
Recent reviews (Pivarnik, 1998; Sternfeld, 1997; Wang, 1998), studies (Koltyn & Schultes, 1997; Ohtake & Wolfe, 1998) and exercise guidelines (Canadian Academy of Sports Medicine, 1998) indicate that moderate exercise during a healthy pregnancy can have beneficial physical and psychological effects on the expectant mother and offer few risks to the fetus.

References

• Canadian Academy of Sport Medicine (1998). Position statement on exercise and pregnancy. Gloucester, Ontario: Author.
• Koltyn, K. F., & Schultes, S. S. (1997). Psychological effects of an aerobic exercise session and a rest session following pregnancy. The Journal of Sports Medicine and Physical Fitness, 37, 287-291.
• Ohtake, P. J., & Wolfe, L. A. (1998). Physical conditioning attenuates respiratory responses to steady-state exercise in late gestation. Medicine & Science in Sports & Exercise, 30, 17-27.
• Pivarnik, J. M. (1998). Potential effects of maternal physical activity on birth weight: Brief review. Medicine & Science in Sports & Exercise, 30, 400-406.
• Sternfeld, B. (1997). Physical activity and pregnancy outcome: Review and recommendations. Sports Medicine, 23, 33-47.

• Canadian Academy of Sports Medicine. Tel: (613) 748-5851, e-mail: jburke@casm.acms.org
• Clapp III, J. F. (1998). Exercising Through Your Pregnancy. Champaign, IL: Human Kinetics
• Exercises During Pregnancy and Postpartum Poster - 16 x 20" full-color poster displays simple exercises. $10.00 US from American College of Obstetricians and Gynecologists. Tel: 1-800-762-2264, website: http://www.acog.org
• Hanton, R. (1996). Times Two: A Prenatal Guide for the Active Women. Ottawa, Ontario: Serious Fun Enterprises.
• Physical Activity Readiness Medical Examination for Pregnancy (PARmed-X for Pregnancy) - a 4-page tear-off form, $2.00/single copy or $9.95/pad of 25 forms from Canadian Society for Exercise Physiology (CSEP). Tel: (613) 234-3755, website: http://www.csep.ca