This document discusses the effects of hyperthermia, or abnormally high body temperature, during pregnancy. It notes that brief fever episodes are not associated with birth defects, but that a core temperature over 102.2°F for 24 hours or more during gestational days 14-28 can increase risks of central nervous system defects in the embryo. The mechanisms by which heat exposure causes defects and the specific defects linked to different gestational periods are described. Guidelines are provided for limiting hot tub exposure to under 15 minutes in 29°C water or 10 minutes in 40°C water to avoid potentially teratogenic temperatures. The document concludes by noting decreased fertility has been linked to occupational heat exposure in both females and males.