“Pregnant women should stay out of saunas.”

The research found this

Most of the worry about sauna in pregnancy comes from fever and from animal experiments. In rats, a 2.5°C rise for an hour during early neural tube closure raised defects of the face and skull. Fever in humans has been linked to developmental defects when a 2°C rise lasted at least a day.1Source 1 · ReviewEffects of heat on embryos and foetusesEdwards MJ, Saunders RD, Shiota K. International Journal of Hyperthermia, 2003.Read the paper ↗PubMed ↗ A pooled analysis of 15 studies found nearly double the odds of neural tube defects (birth defects of the brain and spine) when the mother's body temperature ran high, and most of those studies were about fever. The odds ratio was 1.92, and the plausible range (95% CI) ran from 1.61 to 2.29.2Source 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, women who used a hot tub in the first trimester had 2.8 times the risk of neural tube defects (95% CI 1.2–6.5). For sauna use the figure was 1.8 (0.4–7.9), and that range includes no effect.3Source 3 · Cohort studyMaternal Heat Exposure and Neural Tube DefectsMilunsky A. JAMA: The Journal of the American Medical Association, 1992.Read the paper ↗PubMed ↗ The American College of Obstetricians and Gynecologists (ACOG) cites this paper in exercise guidance that 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. A registry study looked at 100 babies with central nervous system defects and 202 with cleft lip or palate. In all, 98.5% of the mothers had used the sauna regularly, and mothers of affected and unaffected babies had the same sauna habits.5Source 5 · Cohort studySauna and congenital defectsSaxén L, Holmberg PC, Nurminen M, et al.. Teratology, 1982.Read the paper ↗PubMed ↗ A study of 573 babies with heart defects found no link with sauna bathing. Fever of 38°C or more in early pregnancy was more common among their mothers than among the comparison 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 ↗ The baby's heart rate rose by 11–14 beats a minute, and its variability stayed normal. The heat didn't 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 ↗