Q: Are warm water baths and hot tubs safe during pregnancy?

A: There are many studies, both in animals and humans, showing that hyperthermia, the abnormal elevation of body temperature, can adversely effect fetal development. Studies in the the guinea pig and other animals show that an increase of 34.7°F (1.5°C) in maternal body temperature is associated with central nervous system problems and other abnormalities. It’s unclear whether hyperthermia produces adverse effects through direct action on the embryo or by producing maternal toxicity. Other studies have shown that there was a significant increased risk of congenital defects such as neural-tube defects, abdominal-wall defects, and cardiac defects in the babies born to women who reported having a fever during their first trimester.

It has been recommended that maternal body temperature not be repeatedly raised during pregnancy above 102°F (38.9° C). For women using hot tubs or other hot-water treatments, this restriction means limiting exposure to 15 minutes in 102.2°F (39°C) water, or to 10 minutes in 104°F (40°C) to 106°F (41.1·C) water. Sauna baths permit greater heat loss (through evaporation of perspiration) than do water baths, allowing time limits to be extended.