Studies concerning exposure to heat during pregnancy have indicated that maternal hyperthermia can be teratogenic, causing primarily CNS abnormalities. Data, using the animal model, have consistently indicated that the effects of heat are most hazardous when exposure occurs in the first trimester of pregnancy. However, the human data from retrospective studies and sauna bath exposure are not conclusive. Since the risk potentially exists, physicians have been advising expectant mothers to avoid self-inflicted conditions that may result in core temperatures above 38.9 degrees C (102 degrees F). Research has indicated that exercise can result in core temperatures above the recommended level. Considering that early in pregnancy the mother may not appreciate her pregnancy and could exercise at high intensities, the possibility of exposing the fetus to hyperthermia exists. Of the limited studies of exercising pregnant women, there are no data suggesting that normal women actually exercise to a level of exertion that causes significant hyperthermia. However, these studies have been limited to nonathletic populations, in which the exercise has not been prolonged and of high intensity, or sufficient to induce dehydration. Other data indicate that if hyperthermia is a potential consideration for the exercising mother, then exercise in the water may be better as it provides for greater heat loss. The data concerning exposure to cold, although sketchy, suggest that unless the hypothermia is detrimental to maternal survival, there is minimal risk to the fetus.