Hot Tubs & Saunas

Pregnancy

MotherToBaby describes fever and hyperthermia generally, says hot tubs and saunas might cause hyperthermia, and advises limiting their use during pregnancy. Moretti et al. 2005 meta-analyzed 15 studies with 1,719 neural-tube-defect cases and 37,898 noncases; early-pregnancy maternal hyperthermia was associated with neural tube defects (OR 1.92, 95% CI 1.61-2.29), but the exposure was hyperthermia broadly, not hot-tub or sauna use alone. Duong et al. 2011 analyzed National Birth Defects Prevention Study mothers of infants with birth defects (n=10,825) and without defects (n=6,795); any early-pregnancy hot-tub use was associated with gastroschisis (adjusted OR 1.54, 95% CI 1.10-2.17) and anencephaly (adjusted OR 1.68, 95% CI 1.05-2.70). The authors noted multiple statistical tests and small sample sizes and said chance could explain some findings. Chambers 2006 reviewed the older hot-tub/spa literature and noted limited human studies linking early use with neural-tube defects or spontaneous abortion; its abstract gives no study-level estimates or comparison of conflicting results. Ravanelli et al. 2019 reviewed 12 thermoregulation studies (n=347, pregnant participants at any gestational stage); no participant exceeded 39.0°C. The highest mean end-trial core temperature was 36.9°C for hot-water bathing and 37.6°C for sauna exposure. Under the studied conditions, the authors described hot baths at 40°C and hot/dry saunas at 70°C and 15% relative humidity for up to 20 minutes without reaching 39.0°C. These were core-temperature measurements, not fetal or newborn outcome measurements, and do not establish a general exposure limit. ACOG's page says early sauna or hot-tub use, especially for long periods, is associated with birth defects. CDC's 2025 pages list overheating, including getting in a hot tub, as an NTD risk factor and advise avoiding hot tubs or other environments that might cause overheating; they do not provide a pregnancy-specific temperature or duration limit.

Breastfeeding

MotherToBaby states that fever or hyperthermia does not seem to affect the ability to breastfeed, and describes antibodies in milk when fever is caused by illness. The sheet does not present direct study data on planned sauna or hot-tub exposure during lactation. A 2026 Brazilian nationwide cohort included 397,239 children aged 0 to less than 6 months; ambient heat exposure was associated with a 10% higher risk of exclusive-breastfeeding interruption (HR 1.10, 95% CI 1.08-1.14). This measured ambient temperature, not personal hot-tub or sauna exposure, and does not establish a causal effect of those activities. The targeted PubMed search for October 14, 2025 through October 14, 2026 did not identify a direct hot-tub/sauna breastfeeding study among the retrieved items; that search result does not prove that no such study exists.

What was tested

Duong et al. 2011 used a case-control sample from the National Birth Defects Prevention Study and assessed reported maternal hot-tub use in early pregnancy against 17 birth-defect outcomes. Moretti et al. 2005 pooled human studies of maternal hyperthermia and neural-tube defects, not hot-tub or sauna exposure alone. Ravanelli et al. 2019 synthesized 12 studies (n=347) measuring maternal core temperature during exercise, water immersion, hot-water bathing, and sauna exposure; it did not measure fetal outcomes. The 2026 Brazilian cohort assessed ambient temperature and breastfeeding interruption. The 2026 narrative review on bathing and sauna practices excluded pregnant and postpartum participants. No randomized clinical outcome study of hot-tub or sauna use in pregnancy or lactation was identified in the reviewed sources.