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by Dr Jane Moore

August 1, 2002

Swim on, but be careful

In the not too distant past, people viewed pregnancy as a time of confinement. Women expected to drastically limit their activities. With the fitness boom, more and more women began exercising regularly and enjoying it. Since the passage of Title IX, many young women train and compete in strenuous exercise. These women enjoy regular training programs and most do not wish to become completely sedentary when they become pregnant. The American College of Obstetrics and Gynecology (ACOG) reviewed the limited scientific information available in the 1980s. They released a set of guidelines for exercise in pregnancy. These guidelines were quite restrictive because of concerns that regular exercise might cause immediate or long term problems with pregnancies. Active women frequently found that their regular exercise programs quickly exceeded the recommendations.

Both pregnancy and exercise require large changes in body processes involving breathing, heart function, blood circulation and use of energy. There is still concern that the additive effect of some of these changes or the conflicting effects of others may cause immediate or long term problems with a pregnancy. Currently, the research seems to suggest that a healthy woman with an uncomplicated pregnancy may exercise safely during pregnancy without harmful effects on fetal growth and development, pregnancy progression, labor or delivery. Exercise may actually decrease the symptoms of pregnancy. However, it does not appear to provide any advantages or health benefits to the baby.

Most of the changes of pregnancy happen during the first three months. Maternal heart rate increases. Plasma (liquid portion of the blood) volume and the number of red blood cells increase. Stroke volume (amount of blood moved with each beat of the heart) and total cardiac output (total amount of blood pumped by the heart) also increase both at rest and with exercise. The increase in cardiac output with exercise is even greater in swimmers than in other exercisers. With immersion, the hydrostatic pressure of water pushes fluids from the body tissues back into the veins. This occurs very quickly after entering the water and increases the blood volume. This leads to a decrease in blood pressure, an increase in stroke volume and a decrease in heart rate.

The tone of the blood vessels in the arms and legs (peripheral vascular resistance) decreases during pregnancy. Blood flow to the reproductive organs increases greatly. Flow to kidneys, intestines and skin also increases. Exercise typically directs blood flow away from these areas, except the skin, and to the working muscles. Decreasing the blood flow to the uterus could deprive the fetus of oxygen or nutrients and might also cause uterine contractions, leading to premature labor. The intensity and duration of the exercise affect the size of these changes. Regular exercise training decreases the size of these changes. The larger cardiac output in swimmers increases uterine blood flow.

Total lung capacity appears not to change with pregnancy, but the functional residual capacity (the amount of lung space not used during breathing) decreases. The volume of air moved in and out of the lungs per minute (minute ventilation) increases due to an increase in the amount of air taken in with each breath (tidal volume). Respiratory rate does not change. This is true at rest and at all levels of exercise. However, the maximum ventilation during swimming decreases significantly during pregnancy. This appears to be due to hydrostatic pressure of water and the limitations on the timing of breathing imposed by swimming.

Resting oxygen uptake (VO2) increases. This increase is proportional to the increase in body weight with pregnancy. VO2 Max (greatest oxygen uptake with maximum exercise) seems to remain stable for most forms of exercise but appears to decrease in swimming. This decrease may be due to a decrease in breathing capacity. The pressure of the water may push the enlarging uterus and baby upwards, limiting the movement of the diaphragm. A given level of exercise may require a higher percentage of VO2 max later in pregnancy because of body weight increases. Therefore, maximal performance may decrease. Many women spontaneously decrease their level of exercise as pregnancy progresses.

The calories required for a given level of exercise increases. This again is due to the increase in body weight. Non-weightbearing exercise seems to require more calories at a given level than weightbearing exercise. The ability to produce aerobic energy does not change but anaerobic metabolism decreases. During exercise, active muscles use more glucose and divert it away from the fetus. Blood glucose levels fall faster with exercise and to lower levels in pregnancy.

Pregnancy elevates the amount of insulin produced by the pancreas but decreases the sensitivity of the body to its effects. Exercise may limit the development of gestational diabetes by increasing insulin sensitivity. Other endocrine or hormonal changes during exercise are those of a stress response and could possibly interfere with the hormonal changes of pregnancy or start uterine contractions.

Body temperature increases with exercise. The intensity of the exercise directly relates to the increase in body temperature. It increases even more in women with a low fitness level, dehydration, prolonged exercise or a hot, humid environment. If the core body temperature increases above 39-40C in early pregnancy, there may be an increase in birth defects. Later in pregnancy, excess heat may limit fetal growth. Pregnancy does not change sweat production during exercise. More blood pools near the skin in pregnancy because tone in blood vessels decreases. This allows greater heat loss. To offset this, pregnant women produce more metabolic heat. Fetal temperatures stay higher much longer after maternal exercise ends than do maternal temperatures. Water is a much better conductor of heat than air, so swimming in usual pool temperature water (about 80F) results in less increase in core temperature than do land exercises. Swimming in very warm water causes more of an increase in body temperature and should be avoided. It does not appear that most forms of exercise increase the temperature enough to cause problems.

Studies of fetal responses show that changes do occur during maternal exercise. The fetal heart rate increases. This offsets the decrease in blood flow to the uterus and maintains oxygen availability. The fetal heart rate returns to baseline 15 to 30 minutes after exercise, depending on the intensity of the exercise. Occasionally, fetal heart rate decreases during or after exercise. This occurs primarily in unfit women who do rapidly increasing exercise to near maximal levels. This may indicate fetal distress, usually due to placental insufficiency, and is a danger sign. Fetal breathing and movements may be more sensitive indicators of well-being than heart rate. Fetal movements usually increase briefly after moderate exercise.

Exercise does not appear to have an effect on birth weight. Active women do not have more frequent miscarriages, a higher incidence of infertility, more babies with birth defects or more placental abnormalities. Exercise does not appear to cause premature labor and probably has no effect on the length of labor. Exercise does appear to decrease weight gain and fat gain during pregnancy. It maintains maternal aerobic capacity and promotes a sense of self-control and well-being.

The ACOG updated their guidelines in 1994. They are now more liberal and individualized. Regular exercise (at least three times per week) is better than intermittent activity. Highly recommended exercises are swimming, stationary cycling and brisk walking. If there are no risk factors for bad outcomes, a woman can exercise and benefit from mild to moderate intensity exercise. She should monitor heart rate during peak activity. She should avoid extremes of joint flexion and extension and exercise on the back after the first trimester. She should adjust exercise intensity if symptoms develop. She should avoid long periods of motionless standing and exercises where loss of balance could occur. Good warm-up before exercise and cool down afterwards are very important. She should avoid exercises that require jumping, jarring or rapid changes in direction. She should eat foods and drink water adequate for both fetal development and exercise.

Pregnancy, as does training, requires commitment, planning, flexibility and compromise. Anyone needs common sense to develop a sensible, individualized training plan. Women with high blood pressure, anemia, thyroid disease, diabetes, irregular heartbeat, history of early labor, history of slow fetal growth, bleeding or breech presentation during current pregnancy, seizure disorder, excessive obesity, extreme underweight or an extremely sedentary lifestyle are at higher risk of developing problems. They should exercise only with the approval of and close monitoring by their health care provider. Women with history of three or more miscarriages, ruptured membranes, early labor, twins or other multiple fetuses, weak cervix, placenta blocking the cervix, lung disease or heart disease probably should not exercise.

The occurrence of excess fatigue, poor performance or pain indicates the need to modify a training program. Because of our limited knowledge, serious competition with all out efforts should probably be avoided. Moderation and listening to body signals are the most important things to remember. Breathlessness, light-headed feelings, dizziness, aches, pains, exhaustion, irregular or rapid heartbeat, faintness, lower abdominal pain, uterine contractions, vaginal bleeding or loss of fluid and absence of fetal movement are signs of possible problems. If you develop any of these symptoms, you should stop exercising and contact your health care provider.

This month's article is by Jane Moore, M.D., who is a member of the USMS Sports Medicine Committee and 2002 recipient of the Ransom Arthur Award. This article originally appeared in SWIM Magazine, and is reprinted with permission from the author and SWIM Magazine.


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