It has been reported that hyperthermia caused by sauna, hot tub or fever during the early stages of pregnancy in human is a risk factor for birth defects. In hot conditions, body temperature can also rise to fairly high levels. Thus we can surmise that during this period, maternal hyperthermia can also cause fetal anomalies. To investigate this hypothesis, pregnant wistar rats were divided into 3 groups and at 9-11 days of gestation, the first (Experimental) group, animals were immersed in water at a temperature of 40°c for 30 minute. In the second (sham) group, they were immersed in water at 36°C for the same time and the third group were control. On the 18th gestational day, all of fetuses were extracted and examined by cesarean section. In the Experimental group, 30.3% of fetuses had various external anomalies but no anomalies was found in the other groups. Our results show that, hot condition caused the elevation of maternal temperature and resulted in fetal anomalies in rats.