PT on the Net Research

Training the Pregnant Athlete, Part 1: A Summary of the Research

This article is dedicated to Dr. James Clapp, a pioneer investigator in the fields of medicine and exercise physiology. Dr. Clapp studied hundreds of women who exercised five or more times per week, performed weight-bearing exercise for 30-90 minutes at 65-90% of maximum capacity and compared them to healthy, active controls. These women were above average in all fitness parameters and would beconsidered part of the "athlete population." Thank you, Dr. Clapp, for the tremendous job you have done in uncovering the truths behind exercise and pregnancy, for the fit-mom-to-be!


Pregnancy is a normal biological process characterized by the growth and development of a fetus. In this day and age, all mothers want the best possible health for themselves and their babies, but some women have concerns that regular maternal physical activity during pregnancy may cause miscarriage, premature delivery, poor fetal growth, or musculoskeletal injury. For normal pregnancies, these concerns are unsubstantiated and the physiological effects of combining exercise and pregnancy does not support these concerns (Clapp, 1998). In fact, participation in regular weight-bearing exercise has been shown to improve maternal fitness, restrict weight gain without compromising fetal growth, and hasten postpartum recovery.  Thus, a healthy woman with a normal pregnancy may either continue her regular exercise regimen or even begin a new exercise program.

Many of the concerns of exercising while pregnant are centered on theories related to high body temperature, reduced delivery of oxygen and nutrients to the placenta and thus the baby, mechanical stress which may result in damage to the mother or baby and restriction of essential substrates for energy production. The effect of pregnancy on each of these “theories” is discussed with special attention to the numerous benefits, which result from a moderate to vigorous, weight-bearing exercise program. In sum, these benefits include and are not limited to: 

There are more reasons than ever now to continue to follow a training program while one is pregnant. Not only does it benefit the mother, but also the baby. Yes, a baby can be trained in utero.

Dr. Clapp’s research has not been duplicated to date; that is, using an athlete population for investigation. His unique work was based on over 250 case studies of fit women who exercised regularly before, during, and after pregnancy, examining the effects of exercise on the women and their children. Dr. Clapp lays to rest the controversies surrounding pregnancy and exercise, shedding new light on the positive interactions of exercise, health, fitness, and pregnancy. His research enables both expecting mothers and health professionals to develop rational, objective, and individualized approaches to exercise and pregnancy care.  Also researched is the debate surrounding the prescription of strenuous exercise during the process of carrying, birthing, and nursing a baby.

These benefits and the effects of exercise on human physiology will be discussed in the following review piece along with standpoints from other researchers examining the same fit population.

Maternal Benefits of Exercise

The benefits of engaging in a regular, moderate- to vigorous-intensity activities (versus walking and prenatal yoga, which would be considered very low-intensity activities) are ten-fold for the mother-to-be. The research is now clear that the benefits far outweigh any risks, especially when the expectant mother continues her program right to term.

Blood Volume Expansion

The plasma volumes, red cell volumes and total blood volumes of regularly exercising women during pregnancy are at least 10 to 15% higher than those of their less active or sedentary counterparts (Clapp, 1998). What does this mean? Women who exercise have a greater circulatory reserve, which allows them to better handle the stress of exercise and work and also events such as hemorrhage, trauma, anesthesia, and more.

Oxygen uptake

Regular exercise during pregnancy also has some interesting and positive effects on the growth and function of the placenta that helps protect the fetus from oxygen deprivation. Those women who exercise throughout early and mid-pregnancy have a better adapted placenta to deliver more oxygen and nutrients to the baby. There is also an improvement in alveolar ventilation during pregnancy and the muscular effects of regular exercise on ventilation in general, which enhances placental gas transfer of both oxygen and carbon dioxide between the mother and the baby (Clapp, 1998).


It is also known that exposure to regular sustained exercise increases blood volume, which improves an individual’s ability to maintain skin blood flow during exercise and decreases the core temperature threshold for the initiation of the sweat response, meaning pregnant women who exercise regularly will sweat more readily and have a very efficient cooling mechanism (Clapp, 1998). Thermal balance can be maintained with advancing gestation when exercise prescriptions are appropriately modified for conditioned women. (Jones et al., 1985). Although pregnant women should be cautioned to avoid exercise in extremely hot and humid conditions, they are not at risk of “overheating” during exercise.

Body Composition

A woman’s basal metabolic rate increases by 10-15% during pregnancy (Clapp, 1998). However, her ability to store extra energy (calories) is also improved. Thus, it is not necessary to “eat for two” even when training during pregnancy. Women need an additional 300 kcal per day when exercising moderately and if they are exercising more vigorously, about an extra 150 kcal per day on top of that. Interestingly enough, women in third-world countries where they must perform hard, physical work and eat a diet rich in whole, natural, unprocessed food gain less body fat, versus those women in industrialized countries, yet the baby’s birth weights are the same (Clapp, 1998). Following a regular moderate to vigorous, weight-bearing training program is key to reducing maternal body fat accumulation. Again, the examples of walking and prenatal yoga will not be a sufficient enough stimulus to affect body composition.


Pregnancy suppresses the normal hormonal responses that encourage the release of stored liver glycogen when blood sugar levels begin to fall, sparing these carbohydrate reserves for the baby and the placenta. It also decreases the transit time for food to travel through the digestive system, which alters the absorption rate of this fuel source in the blood (Clapp, 1998). The two of these effects combined, can cause pregnant women to feel mildly diabetic if they do not replenish fuel stores after training or if they go more than 6-8 hours without food. Thus, it is important to have, on hand, a small piece of fruit and a protein source to ingest after exercise, or to keep exercise sessions shorter than normal to keep blood sugar levels from dropping too low. However, those pregnant women who are more highly-trained and are used to more vigorous training sessions have an increased reliance on fat for energy, which allows glucose to be more readily available to the baby. Fit, pregnant women maintain more constant blood glucose levels versus unfit, generally active controls.

Less Maternal Discomfort

When data was reviewed from 100 women, the incidence of low back, pelvic and or leg discomfort in women who exercised during pregnancy was less than 10 percent. In the control group of 100 women who were active, but not trained, the incidence was much greater, at 40% (Clapp, 1998). It was not known however what types of exercise these exercising women were doing however beyond those that were regular, weight-bearing and sustained.

Labor and Delivery Benefits

In the research, it has been found that continuing weight-bearing exercise at the same intensity, duration and frequency throughout the entire pregnancy leads to a 35% decrease in the need for pain relief, a 75% decrease in the incidence of maternal exhaustion, a 50% decrease in the need to artificially rupture the membranes, and a 75% decrease in the need for operative intervention such as a cesarean section or forceps delivery (Clapp, 1998). What was most interesting is that a whopping 86% of the exercise group in Dr. Clapp’s study had uncomplicated, spontaneous deliveries versus only 53% of the control group of the active, but untrained, women. Also, among the women who had vaginal births, the length of labor was more than a third shorter in the women who continued to exercise right to term versus the controls (Clapp, 1998).

Effects on the “Intangibles”

In general, women who continue to train during their pregnancy have a positive attitude and seem ready to take on the challenges of labor. Dr. Clapp describes his fit research subjects’ attitudes by saying “they regard these events as a normal part of life and therefore take them in stride” (Clapp, 1998). Simply put, they looked on the bright side of things and used humor as a way to cope with their changing bodies. According to Dr. Clapp, his control group did not share the same upbeat attitude.

Fetal Well-Being

While it is wonderful to share all the positive adaptations and benefits that come with pregnancy and exercise for the mother, it is also important to discuss the effects on the fetus and examine how he or she might be dealing with the stresses of physical activity. It is also important to highlight the benefits of exercise on survival value for the baby when unanticipated maternal stresses occur during labor.

Fetal Heart Rate (FHR) Response

The normal and usual fetal heart rate response is an increase in heart rate during exercise that gradually returns to its pre-exercise level once the exercise session ends (Clapp, 1998; Avery, 1999). There are some conditions, however, that need to be met for the fetal heart rate increase to occur. First, the exercise must be sustained for more than 10 minutes in duration. Also, the type of exercise chosen affects the magnitude of the fetal heart rate response. For example, exercises that use a large fraction of muscle mass and force the woman to move her body weight against gravity such as: aerobics, stair climbing, jogging, sled pushing/pulling result in a greater FHR increase. Both longer and more intense exercise sessions do decrease uterine blood flow and the fetus compensates for this by increasing its heart rate. This of course, poses the question then of how much stress is necessary and how much is too much for the fetus? Is it safe for the FHR to increase 25-35 beats per minute above the pre-exercise level?

Dr. Clapp and his investigators found there were no changes in fetal breathing or activity patterns, characteristic of a lack of oxygen, with elevated heart rates. Instead, they concluded that heart rate increase was a normal stress response to the exercise, rather than a physiological reaction to insufficient oxygen.

There is also another response the fetal heart rate can undergo with exercise of sufficient magnitude and that is a drop or decreased FHR response. This can happen between 15 and 20% of the time in UNFIT women, exercising close to their maximum, as shown by other investigator’s research. In Dr. Clapp’s studies of fit women, exercising at above 85% of their capacity (not to maximum) it has only happened in 2 out of 2000 cases, very close to term. In both cases, the baby was perfectly healthy. Dr. Clapp hypothesizes that the fall in FHR response was due pressure on their heads from the mother’s pelvic structures, not lack of oxygen.

Dr, Clapp advises monitoring fetal kicks for 30 minutes after exercise. If there are two or more kicks, the baby’s condition is fine and there is nothing to worry about.

Exercise Prescription and Monitoring

There is no replacement for common sense with respect to exercise programming during pregnancy. There are many resources that can be accessed when putting together a plan for oneself, a client, or a patient. It is not necessary to adhere to a set number of exercises, repetitions, or intensity as each and every person is different and will have a different training background. In fact, a training program is only as good as the subject’s adaptive ability. So, in essence, it is more important to monitor day-to-day responses to the exercise, than try to plan something out far in advance. The overall exercise program must be congruent with an individual’s goals and their lifestyle. One should cycle rest with training and note that pregnancy is not a time to reach elite levels of performance. It takes energy to develop a healthy baby.

Guidelines for prescription and monitoring as well as “Do’s and Don’ts” can be found at:

“The 1994 bulletin of the American College of Obstetricians and Gynecologists indicates that “[t]here are no data in humans to indicate that pregnant women should limit exercise intensity and lower target heart rates because of potential adverse effects.”

The American College of Obstetricians and Gynecologists (ACOG) no longer places restrictions on heart rate during aerobic sessions. Larger, more comprehensive studies on aerobic exercise during pregnancy have shown that women do not need to limit their sub maximal heart rate. Complete current ACOG guidelines for exercise during pregnancy can be accessed on their website.”

Unfortunately, there is very little research on the effects of resistance training, but Dr. Clapp does encourage it. One will not find much useful information on advanced strength training prescription in prenatal course curriculum or the Exercise Physiology literature as they do not have expertise in this area. Instead, it is recommended that the expectant mother seek a strength and conditioning coach, who specializes in technical lifting. He or she will be able to modify exercises and develop a program that promotes a strong core, great posture and muscle balance.

It is also important to note that athletes and very fit mom-to-be’s need to mentally downshift in the last half of pregnancy. Most athletes have developed the ability to push through the pain and ignore internal cues to either stop or slow down. Many are intensely competitive and ambitious in their physical goals. While these levels of motivation often enhance athletic success, they also greatly increase the risk of injury during the last half of pregnancy. It is important to be aware of this mentality whether you are the athlete or the coach. Pregnancy is not the time to reach personal bests. Instead it is the time to put the development of the baby first to always listen to one’s body. If something feels uncomfortable or foreign, it is always wise to stop and inform your physician or midwife right away.

In Part Two of this article, a panel of experts give their insight and perspectives on the myths surrounding pregnancy and training, important tips for moms-to-be as well as inside information not found readily in the prenatal literature.


  1. American College of Obstetricians and Gynecologists (1994). Exercise During Pregnancy and the Postpartum Period. Technical Bulletin No 189, ACOG: Washington, DC.
  2. Clapp, J.F. (1998). Exercising through your pregnancy. Human Kinetics Publishers: Champaign, IL.
  3. Clapp, J.F. 3rd & Little, K.D. (Jul 1995). The Interaction Between Regular Exercise and Selected Aspects of Women`s Health. Am J Obstet Gynecol 173(1):2-9.
  4. Clapp J.F. 3rd, et al. (Jan 2002) Continuing Regular Exercise During Pregnancy: Effect of Exercise Volume on Fetoplacental Growth. Am J Obstet Gynecol 186(1):142-7.
  5. Avery, N.D., et al. (1999). Fetal responses to maternal strength conditioning exercises in late gestation. Canadian Journal of Applied Physiology 24(4) : 362-376.
  6. Jones R.L., Botti J.J. & Anderson WM, Bennett N.L. (1985). Thermoregulation During Aerobic Exercise in Pregnancy. Journal of Obstetrics and Gynecology 65 (3).