The information contained in this article applies only to female athletes who are very fit and participated regularly in sports before becoming pregnant. This information does not apply to women who are new to exercise or have not participated in high-intensity exercise prior to pregnancy. There is good evidence that starting such activity for the first time while pregnant can be dangerous to both the woman and the fetus.
A Little Background
In 1985, the American College of Obstetricians and Gynecologists (ACOG) published its guidelines for prenatal exercise, cautioning women not to exceed a maximum heart rate of 140 beats per minute (bpm) (ACOG 1985). With these guidelines, pregnant athletes found themselves in a quandary, since 140 bpm hardly qualifies as a workout for most fitness enthusiasts. In 1994, ACOG revised its guidelines, making them less restrictive but still failing to address the upper limits of exercise (ACOG 1994).
Researcher and physician James Clapp, MD, was one of the first medical experts to break rank with these conservative guidelines. He took the position that athletes are more efficient at handling the stresses of exercise during pregnancy and can safely exercise beyond the ACOG guidelines if properly trained and monitored. His book, Exercising Through Your Pregnancy (2002), is an excellent resource for the fitness professional with athletic clients and the source for many of the less conservative recommendations that follow in this article.
Effects of Exercise on the Fetus
When it comes to the effect of exercise on the fetus, the three areas of concern are hyperthermia, sports injuries and oxygen deficit.
Hyperthermia. The most common concern for the fetus during high-intensity exercise is hyperthermia, a higher-than-normal body temperature. Exercise dramatically increases body temperature, and the fetus can take on the mother's heat, possibly leading to birth defects. Yet to date, research has not proven any increase in birth defects when women exercise at high intensities (ACOG 1994). In fact, studies indicate that fit clients actually have a better ability to dissipate heat (ACOG 1994).
It is during the first trimester that the fetus cannot regulate its own body temperature and is most susceptible to the mother's. In this period, pregnant athletes should be cautious about exercising in hot conditions and for long durations. They should wear light-colored, breathable fabrics to keep cool and should drink water throughout the day and during exercise bouts; their urine should be diluted to the point that it is virtually clear in color.
Some experts recommend that pregnant athletes take their temperature either vaginally or rectally (orally is less accurate) immediately before their longest weekly workout and again immediately after, before the body cools down. Clapp recommends a temperature increase of no more than 1.6 degrees Celsius (3 degrees Fahrenheit [F]) and a post-exercise temperature no higher than 102 degrees F (Clapp 2002).
Sports Injuries. At any stage during pregnancy, an extreme blow to (or fall onto) the abdomen can damage the placenta. Later in pregnancy, as the fetus moves higher in the womb and is unprotected by the pelvis, there is greater risk of damage to the fetus itself by direct impact during sport (Sports Medicine Australia [SMA] 2001).
Most medical experts agree that the kinds of falls and direct contact that typically occur during contact sports are unlikely to damage either the womb or the fetus. However, because there is some potential for injury, it is ultimately up to each client and her physician to decide which sports are safe to play during pregnancy. Here are some specific sports that I have found can be problematic for pregnant athletes. I usually discourage my own clients from participating in these activities:
- Contact sports such as hockey (field and ice), boxing, wrestling, football and soccer, all of which increase the risk of abdominal trauma
- High-risk sports, such as gymnastics, horseback riding, skating (ice, roller and inline), skiing (water and snow), hang gliding, racquetball and scuba diving, all of which increase the risk of falls/trauma to the fetus
Oxygen Deficit. The duration, type and intensity of the exercise that pregnant athletes perform can all affect the fetus's heart rate. However, if a pregnant athlete trains regularly, the fetus will be better conditioned and able to adapt to the stresses of exercise. The concern arises when a woman performs extreme levels of exercise on an inconsistent basis: Then, the effect can be detrimental to the fetus.
Studies have shown that when unfit women exercise at very high intensity levels, uterine blood flow decreases to the point where the fetus experiences a serious oxygen deficit (Wolfe & Mottola 1994). Fortunately, this rarely occurs when the pregnant mother is fit or an athlete.