Raising your core body temperature (hyperthermia) can harm your fetus, particularly during the early weeks of organ development. While experts don’t forbid hot tub or sauna use, they do advise caution.
Hyperthermia during the first weeks of fetal development has been linked to neural tube defects. So pregnant women are advised to treat a high fever with acetaminophen and to avoid other causes of hyperthermia.
There is no firmly established temperature or length of exposure that is considered safe during pregnancy. If you use a hot tub or sauna during pregnancy, be conservative. Avoid uncomfortably high temperatures, and limit your exposure.
The American Academy of Pediatrics and the American College of Obstetricians and Gynecologists suggest sauna use of no more than 15 minutes and hot tub use of no more than 10 minutes. To help keep your body temperature down in a hot tub, sit with your arms and upper torso above the water.