There are a limited number of human studies linking hot tub or spa use during early pregnancy to increased risks for neural tube defects (NTDs) or spontaneous abortion. However, these data can be considered in the context of human studies that have demonstrated an association between high maternal fever in early pregnancy and NTDs. In addition, there is a large volume of animal literature suggesting that, regardless of the heat source, an elevated core maternal temperature at or above the threshold of 2°C over baseline, as well as timing and duration of exposure, are the critical factors in conferring risk. Therefore, the potential for hot tub or spa use to increase core maternal body temperature to risky levels and thus increase the risk for NTDs is likely. A woman who knows or who may not yet be aware that she is pregnant should be advised of the recommended limits of exposure. She should also be aware of the possible variability in hot tub or spa temperature readings and be able to accurately monitor maximum water temperature in the hot tub or spa so that her body temperature can be maintained below 38.9°C. Birth Defects Research (Part A) 76:569–573, 2006. © 2006 Wiley-Liss, Inc.