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...