“Pregnant women should stay out of saunas.”

The research found this

Concern about sauna in pregnancy comes mostly from fever and from animal experiments. In rats, a 2.5°C rise for an hour during early neural tube closure raised craniofacial defects, and in humans a 2°C rise lasting at least a day during fever has been linked to developmental defects.1Source 1 · ReviewEffects of heat on embryos and foetusesEdwards MJ, Saunders RD, Shiota K. International Journal of Hyperthermia, 2003.Read the paper ↗PubMed ↗ A meta-analysis of 15 studies found an odds ratio of 1.92 (95% CI 1.61–2.29) for neural tube defects after maternal hyperthermia, and most of those studies were about fever.2Source 2 · Meta-analysisMaternal Hyperthermia and the Risk for Neural Tube Defects in OffspringMoretti ME, Bar-Oz B, Fried S, et al.. Epidemiology, 2005.Read the paper ↗PubMed ↗

The largest study that reported sauna separately followed 23,491 women in New England who were having prenatal screening. After adjustment, first-trimester hot tub use carried a relative risk of neural tube defects of 2.8 (95% CI 1.2–6.5), and sauna use 1.8 (0.4–7.9), which was not statistically significant.3Source 3 · Cohort studyMaternal Heat Exposure and Neural Tube DefectsMilunsky A. JAMA: The Journal of the American Medical Association, 1992.Read the paper ↗PubMed ↗ ACOG cites this paper when its exercise guidance links heat exposure to neural tube defects.4Source 4 · GuidancePhysical Activity and Exercise During Pregnancy and the Postpartum PeriodAmerican College of Obstetricians and Gynecologists, Committee on Obstetric Practice. American College of Obstetricians and Gynecologists, Committee on Obstetric Practice, 2020.Read the paper ↗

Finland offers a natural test, because nearly every pregnant woman there used the sauna. In a registry study of 100 babies with central nervous system defects and 202 with orofacial clefts, 98.5% of mothers had used the sauna regularly, and sauna habits were the same in case and control mothers.5Source 5 · Cohort studySauna and congenital defectsSaxén L, Holmberg PC, Nurminen M, et al.. Teratology, 1982.Read the paper ↗PubMed ↗ A study of 573 heart malformations found no link with sauna bathing, while fever of 38°C or more in early pregnancy was more common in case mothers (10% against 6%).6Source 6 · Cohort studyMaternal hyperthermia during pregnancy and cardiovascular malformations in the offspringTikkanen J, Heinonen O. European Journal of Epidemiology, 1991.Read the paper ↗PubMed ↗

Researchers in Turku measured what a moderate sauna does. Twenty minutes at 70°C and 15% humidity raised rectal temperature by 0.3–0.4°C in early and late pregnancy.7Source 7 · Clinical trialHaemodynamic Response to Moderate Thermal Stress in PregnancyVähä-Eskeli KK, Erkkola RU, Seppänen A, et al.. Annals of Medicine, 1991.Read the paper ↗PubMed ↗ Fetal heart rate rose by 11–14 beats a minute and its variability stayed normal, and the heat did not start labor in any of the 23 women tested late in pregnancy.8Source 8 · Clinical trialThe effect of short-term heat stress on uterine contractility, fetal heart rate and fetal movements at late pregnancyVähä-Eskeli K, Erkkola R. European Journal of Obstetrics & Gynecology and Reproductive Biology, 1991.Read the paper ↗PubMed ↗9Source 9 · Clinical trialDoppler Flow Measurement of Uterine and Umbilical Arteries in Heat Stress During Late PregnancyVähä-Eskeli K, Pirhonen J, Seppänen A, et al.. American Journal of Perinatology, 1991.Read the paper ↗PubMed ↗ A systematic review of 12 studies with 347 pregnant women found no one above a core temperature of 39.0°C.10Source 10 · Systematic reviewHeat stress and fetal risk. Environmental limits for exercise and passive heat stress during pregnancy: a systematic review with best evidence synthesisRavanelli N, Casasola W, English T, et al.. British Journal of Sports Medicine, 2019.Read the paper ↗PubMed ↗

Guidance differs by country. ACOG advises avoiding high heat and humidity above all in the first trimester, and a retired NHS page said women may choose to avoid saunas because of the risk of overheating.4Source 4 · GuidancePhysical Activity and Exercise During Pregnancy and the Postpartum PeriodAmerican College of Obstetricians and Gynecologists, Committee on Obstetric Practice. American College of Obstetricians and Gynecologists, Committee on Obstetric Practice, 2020.Read the paper ↗11Source 11 · GuidanceIs it safe to use a sauna or jacuzzi if I'm pregnant? (Common health questions)NHS. NHS, 2022.Read the paper ↗ Finland's Duodecim health library says healthy pregnant women can take saunas without worry and advises women with high blood pressure in pregnancy to consult their doctor.12Source 12 · GuidanceSauna ja terveys [Sauna and health]Viinikka L. Lääkärikirja Duodecim, Terveyskirjasto, 2024.Read the paper ↗