Exercise at high temperature causes maternal hyperthermia and fetal anomalies in rats.

Hyperthermia is thought to be a teratogen in many animal species and also in humans. It has been reported that hyperthermia caused by sauna, hot tub, or fever during the early stages of pregnancy is related to an increased risk for neural tube defects. During exercise, especially in hot conditions, body temperature can also rise to fairly high levels. Thus, we can surmise that hyperthermia induced by exercise can also cause fetal malformation. To investigate this hypothesis, pregnant rats at 9 days of gestation were divided into four groups. In the first group, the animals were made to swim for 30 minutes in water at a temperature of 40.5 degrees C. In the second group, they were restrained and immersed in water for the same time at the same temperature. In the third group, the rats were forced to swim in water at 36.0 degrees C. The fourth group were controls. The core temperature of the rats was measured during these procedures. On the 18th gestational day, fetuses were extracted by cesarean section. The elevation of maternal core temperature was significantly greater in the first group than in the other groups. In the first group, 69% of fetuses had various external anomalies. No anomalies were found in the other groups. Our results show that exercise in hot conditions caused the elevation of core temperature and resulted in fetal anomalies in rats.