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Exercising Through Your Pregnancy

Addicus Books, Inc.

How Exercise Affects Pregnancy

I became interested in the effects of exercise during pregnancy some thirty years ago because many women chose to continue exercising during their pregnancies, yet we knew little about the effect it might have on the baby or the woman. At that time, there were two schools of thought. The conservative school, which included most health care providers, felt that exercise during pregnancy was potentially harmful and therefore recommended a restrictive, cautious approach to exercise for healthy pregnant women. This approach sprang from the finding that a variety of maternal lifestyle factors could compromise pregnancy outcome and the knowledge that several physiological changes induced by exercise could potentially harm a pregnancy.

The liberal school was represented by young women who had exercised regularly during one or more of their pregnancies. These women felt that strenuous physical activity during pregnancy was not only normal but also helpful and they recommended it to healthy women to improve the course and outcome of their pregnancies. This view had its origins in historical perspective and anecdotal or personal experience.

These polarized views did two things. First, they generated conflict among active women, their health care providers, and, often, their friends and families as well. Second, they stimulated many different groups to begin to study the effects of exercise during pregnancy.

Beginning the Research

Because there were many theoretical concerns, little factual knowledge, and a growing number of women exercising vigorously during pregnancy, I decided it was time to study some veteran women competitive runners and aerobic dance instructors who maintained their exercise regimens throughout pregnancy. By studying these women, we would determine if the frequent (five or more times per week), prolonged (thirty- to ninety-minute) bouts of high-intensity (65 to 90 percent of maximum capacity) weight-bearing exercise had any effect on the course and outcome of their pregnancies. Fortunately, our initial studies were designed to include objective measurements of many factors to avoid missing something important the first time around.

Analyzing the Research

These studies required an immense amount of time and effort for everyone concerned, but it quickly paid off. When we began to analyze the information from the first ten of these women, we were surprised and excited. Becoming pregnant had changed these women's bodily responses to their regular exercise routines. During very early pregnancy, their heart rates suddenly went sky-high, both at rest and during exercise. This reaction was so early and dramatic that it alarmed several of the women, but it turned out to be simply an early, previously unrecognized sign of a healthy pregnancy.

Later in the pregnancy, the heart rates of these women during exercise came back down. By late pregnancy, it was hard for most women to get their exercise heart rates up to the levels recorded before pregnancy, even though their workloads were the same or higher. Energy requirements during exercise also decreased, indicating that their metabolic efficiency had improved. Finally, during pregnancy, their blood sugar levels fell during and after exercise, which was the reverse of what happened before they became pregnant.

These unexpected and dramatic changes were exciting because they meant that understanding the effects of exercise on the course and outcome of pregnancy might be straightforward. It looked as if many functional changes induced by the hormonal signals of pregnancy had modified various aspects of the exercise response in a manner that would protect the unborn baby.

It also appeared that exercise-induced cardiovascular and metabolic training effects enhanced the functional changes of pregnancy in a manner that was also protective. This information provided a train of thought proved to be naive, but it did provide a vital element of early understanding, helping us plan the experiments that eventually confirmed that very fit active women could maintain this level of physical activity throughout pregnancy without harm.

Over the next several years, we conducted many experiments to improve our understanding of the interaction between the changes induced by pregnancy and those induced by exercise.

Understanding How Exercise Affects Pregnancy

Eventually, we identified several critical factors at work in the interaction that led to an understanding of how regular, frequent, sustained, and moderate- to high-intensity, weight-bearing exercise influences the course and outcome of pregnancy for mother and baby. (Frequent exercise was considered more than three times per week. Sustained was considered twenty minutes or more per session.) In turn, this enabled us to develop some rational principles to use as guidelines in designing individualized exercise programs for pregnant women.

Indeed, once the basic principles underlying the physiological interaction between regular exercise and pregnancy are understood, it's easy to design an appropriate, individualized exercise regimen for any healthy woman who is either considering pregnancy or already pregnant. The remainder of this chapter is designed to start toward that goal.

How the Heart and Circulatory System Adapts to Pregnancy

Pregnancy affects the function of a woman's heart and circulation at many levels and in many ways. These changes are mediated by hormonal signalss from the embryo, fetus, and placenta. During pregnancy, the entire circulatory system changes dramatically to support the needs of the woman's body and the increasing needs of her developing child

Unfortunately, these changes also cause many of the unpleasant symptoms of pregnancy such as lightheaded-ness, nausea, unbelievable fatigue, cravings, constipation, bloating, frequent urination, and others.

These adaptations actually begin very early, at or about the time the fertilized egg implants in the wall of the womb. The outer rim of cells destined to become the placenta initiate the changes by releasing hormonal signals, which initiate relaxation and reduced responsiveness in most, if not all, the muscle cells in a woman's blood vessels. The result is that both the elasticity and volume of the entire circulatory system (heart, arteries, and veins) increases virtually overnight.

This creates a big problem: suddenly there is not enough blood in circulation to fill it up. The amount of blood returning to the heart decreases, as does the amount of blood in the heart and the amount the heart pumps out. As a result, blood pressure falls, especially when the pregnant woman stands up. From the change in the amount of blood in the heart and the lower blood pressure, the body senses that the vascular system is underfilled. In response, it triggers the release of several hormones from the heart and adrenal gland, which cause the body to decrease the excretion of salt and water by the kidneys. The retained extra salt and water rapidly expands the volume of plasma in the vascular system, correcting the underfill problem, and allows more blood to return to the heart so it can pump more out (cardiac output), thereby improving arterial pressure and blood flow to the organs. Eventually, the responses to the initial hormonal signals result in average increases in heart volumes (chamber volume and stroke volume, which is the amount of blood pumped with each beat) of 15 to 20 percent. Both blood volume and cardiac output increase by about 40 percent.

This all takes time, however. In the meantime, the woman may experience all the symptoms of vascular underfill such as:

• waves of sudden fatigue

• a racing pulse

• nausea

• pallor

• sweating

• dizziness, especially when getting up quickly or during standing

At first these symptoms are scary, but they decrease progressively as the blood volume expands and usually are gone by the end of the fourth month.

The final circulatory adaptation to pregnancy is a blunted release of the stress hormones epinephrine and norepinephrine in response to a variety of stresses, including exercise. The blood vessels' responses to those hormones and similar drugs also are depressed.

These adaptations eventually convert a relatively high-resistance, average-volume, normal-flow-rate circulatory system into a low-resistance, high-volume, high-flow — one needed to maintain the growth and development of the fetus within the body of the mother. The pressure in the arteries remains relatively low because the increase in the amount of blood pumped by the heart is still not great enough to match the degree of relaxation and dilation of the blood vessels that occurs. The vascular relaxation and dilation are most pronounced in the blood vessels that supply the skin, kidneys, and reproductive tissues. As a result, a large fraction of the additional blood available goes to these tissues, and their blood flows increase dramatically (two- to twentyfold) higher than before pregnancy.

These local changes in blood flow protect the baby but they also cause some bothersome symptoms for the pregnant woman. For example, the increase in skin blood flow raises skin temperature. While this improves a woman's ability to dissipate heat, it also makes her feel warm and appear flushed (especially in the palms and face). The increase in kidney blood flow improves waste removal, which ensures that the kidneys can handle the increased load of metabolic waste associated with the baby's growth. However, it also results in an increased volume of urine, which, along with pressure from the enlarging womb, stimulates frequent urination (a major problem for runners.) Finally, the increased flow to the reproductive tissues ensures adequate delivery of oxygen and nutrients to the developing placenta and baby, but it also creates the uncomfortable sensation of pelvic and lower abdominal fullness.

How the Heart and Circulatory System Adapts to Exercise

Many classic studies that identified the circulatory adaptations to exercise were done in the late 1960s and early 1970s. They clearly demonstrated that regular, vigorous exercise training increases blood volume, the size of the heart chambers, the volume of blood pumped with each beat, and the maximum cardiac output that can be achieved. It also increases the density and growth of blood vessels within skeletal muscle and the number of elements within the muscle cells that generate energy.

In addition, it improves an individual's ability to dissipate heat by increasing the ability to sweat and lowering the temperature required to produce an increase in skin blood flow. These changes improve cardiovascular capacity, exercise capacity, and efficiency in many ways. For example, the need to shift blood flow away from the internal organs to the muscle during exercise is reduced, as are the heart rate, blood pressure, and thermal responses to any physical task. I'm sure you have noticed that five of these adaptations are similar to those induced by the hormonal signals of pregnancy. These include increases in the following:

• the volume of blood in the circulation

• the skin blood flow response

• the size of the heart chambers

• the volume of blood pumped each beat

• the delivery of oxygen to the tissues

Interactive Effects

As you might have already guessed, the changes produced by regular weight-bearing exercise actually complement those induced by pregnancy. Indeed, the circulatory status of a normal pregnant woman at rest has many similarities with that of a trained nonpregnant woman during exercise (volume expanded, hyperdynamic, high blood flows to tissue, and so on).

Moreover, it should be no surprise that when fit women maintain their exercise regimen during pregnancy, the cardiovascular adaptations to pregnancy are superimposed on their preexisting adaptations to training. The results of the interaction are at least additive. For example, the plasma volumes, red cell volumes, and total blood volumes of regularly exercising women during pregnancy are at least 10 to 15 percent higher than those of their sedentary sisters. This means that women who exercise regularly during pregnancy have more circulatory reserve, which improves their ability to deal with both anticipated (exercise, work) and unanticipated (hemorrhage, trauma, anesthesia, and so forth) circulatory stress.

Likewise, in the active woman, pregnancy enhances the exercise-induced increases in left ventricular volumes. As a result, the amount of blood pumped by the heart each beat is 30 to 50 percent greater than that of a healthy but sedentary woman.

The only potential conflict between the circulatory demands of exercise and those of pregnancy is where the blood goes. During exercise it goes to supply the heart, muscles, skin, and adrenal glands, with a decrease in the flow to the renal, gastrointestinal, and reproductive systems. During pregnancy it goes to supply the reproductive tissues, kidneys, and skin, without significantly changing the rate of blood flow to other structures. From a safety point of view, the question is whether the cardiovascular adaptations in the fit, exercising, pregnant woman are sufficient to simultaneously maintain adequate blood flow and oxygen delivery to the exercising muscle and the fetus. As I discuss in later chapters, recent data indicates that, under most circumstances, the correct answer is the affirmative one.

Interpreting Exercise Heart Rates during Pregnancy

One of the most confusing aspects of monitoring exercise intensity during pregnancy is whether to use heart rate response as a guideline. Before pregnancy, women who exercise regularly often use their heart rate response to exercise as a training intensity guide. To be sure that they are achieving a reasonable training effect from their exercise without risk, these women may determine their target heart rate range and work to keep their heart rate in this range. They determine their target heart range from a chart or calculate it as the range between 70 and 85 percent of their maximum heart rate.

Exercising women quickly notice that their heart rate response to exercise changes during pregnancy and wonder why. Often, I've found the questions women ask about their heart rates express guarded concern and usually deal indirectly with issues of safety, health, or fitness. For example, the question, "Why does my heart rate go over 180 when I do aerobics?" really means, "Is it safe for the baby to let my heart rate go that high?"

The answer to each of these questions, as well as most others, is that it depends. Figure 1.2 details the possible reasons why heart rate is not a good predictor of how hard a woman is working during pregnancy and therefore is not a reliable measure of safety, health, or fitness. In fact, she probably is better off not monitoring it unless she knows a lot about her heart rate and its response to exercise before pregnancy.

Indeed, it turns out that a pregnant woman's perception of how hard she is working using the Borg Rating of Perceived Exertion (RPE) scale may be a much better index of exercise intensity than her heart rate. Borg's scale allows the individual to numerically rate how hard she feels she is working, and it probably is the best way to monitor exercise intensity during pregnancy. I'll discuss how to use the Borg RPE scale in chapter 2.

Pregnant or not, there are several reasons why relying solely on heart rate response during exercise is not the safest or most appropriate way to determine exercise intensity. Although there is a linear relationship between the intensity of exercise and the heart rate response in all individuals, the resting heart rate, slope of the heart rate response, maximum heart rate, environmental factors, and measurement technique vary from one individual to another. Some reasons you should not rely solely on heart rate response include the following:

• A woman's genetic makeup can create a fifteen- to thirty-beat-per-minute difference in her heart rate while exercising at a moderate to high intensity. Thus, individuals with low resting heart rates have lower heart rates at any exercise intensity, and vice versa.

• A twenty-year-old's exercise or target heart rate can easily be ten to twenty beats per minute higher than a thirty-five-year-old's at the same exercise intensity.

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Excerpted from Exercising Through Your Pregnancy by James F. Clapp III, Catherine Cram, Ken Halfmann. Copyright © 2012 James F. Clapp III, M.D., and Catherine Cram, M.S.. Excerpted by permission of Addicus Books, Inc..
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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