Sauna and Pregnancy: What the Research (and ACOG) Actually Say (2026)
By IceColdTubs · Updated August 25, 2026
Quick answer: Sauna use during pregnancy isn’t a flat “no,” but it isn’t a flat “fine” either — ACOG’s core guidance is to avoid a sustained core body temperature above 38.9°C (102°F), a threshold saunas can reach. The most-cited neural-tube-defect study (Milunsky et al., JAMA, 1992, n=23,491) found sauna-specific risk wasn’t statistically significant (crude RR 2.6, 95% CI 0.7-10.1), while hot tub exposure was (RR 2.9, 95% CI 1.4-6.3) — an important distinction most sauna-and-pregnancy content glosses over. Risk is concentrated in the first trimester, when the neural tube closes around week 6; any sauna use later in pregnancy should still get a doctor’s sign-off first, with strict time and temperature limits.
Search “sauna during pregnancy” and the results split into two extremes: alarmist posts implying any heat exposure risks birth defects, and Finnish-culture posts implying it’s basically fine because Finnish women have always done it. The actual research sits in the middle, with real nuance that gets lost in both directions.
The core-temperature threshold ACOG actually uses
The American College of Obstetricians and Gynecologists frames the risk around a specific number, not a blanket ban: avoid activities or environments that raise and sustain core body temperature above 38.9°C (102°F), particularly during the first trimester when the embryo’s organ systems — including the neural tube — are forming. That threshold is the actual mechanism of concern. A traditional sauna at 80-90°C ambient air can push core body temperature past that line within minutes if a session runs long, which is why time and temperature limits matter more than the sauna type itself.
What the birth-defect research actually found (and didn’t)
The most frequently cited study on this topic is Milunsky and colleagues’ 1992 prospective cohort in JAMA (268(7):882-885), which followed 23,491 women who underwent serum alpha-fetoprotein screening or amniocentesis, tracking self-reported hot tub, sauna, fever, and electric blanket exposure in early pregnancy against neural tube defect outcomes.
| Heat source | Crude relative risk | 95% CI | Statistically significant? |
|---|---|---|---|
| Hot tub | 2.9 | 1.4 – 6.3 | Yes |
| Sauna | 2.6 | 0.7 – 10.1 | No |
| Fever | 1.9 | 0.8 – 4.1 | No |
| Electric blanket | 1.2 | 0.5 – 2.6 | No |
After adjusting for maternal age, folic acid supplementation, and family history of neural tube defects, hot tub exposure remained significant (adjusted RR 2.8, 95% CI 1.2-6.5), while sauna’s adjusted RR (1.8) stayed non-significant. That’s a genuinely important distinction: the single most-cited study behind “saunas cause birth defects” headlines actually found its strongest, statistically significant signal for hot tubs — sauna’s own numbers pointed the same direction but the confidence interval was too wide (crossing 1) to call it a proven effect in this sample.
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The more recent, more reassuring evidence — with a real caveat
A 2019 systematic review by Ravanelli and colleagues in the British Journal of Sports Medicine, “Heat stress and fetal risk,” pooled 12 studies covering 347 pregnant women exposed to passive heat stress or heat-related exercise, using ACOG’s 39.0°C core-temperature ceiling as the safety marker. Across every exposure protocol tested, no participant’s core temperature exceeded 39.0°C — evidence that pregnant women’s thermoregulation may be more protective than the older, more alarmist framing assumed.
The caveat matters: this review pooled relatively short, controlled heat exposures across various protocols — it isn’t a specific green light for long, high-heat traditional sauna sessions, and the authors’ conclusion was that a blanket “avoid all heat exposure” recommendation may be overly broad, not that saunas carry no risk. It supports a middle position — moderate, time-limited, doctor-approved sessions — rather than either extreme.
Trimester by trimester
| Trimester | General guidance | Why |
|---|---|---|
| First (weeks 1-12) | Avoid sauna entirely | Neural tube closes by ~week 6; highest-risk window per Milunsky et al. and ACOG |
| Second (weeks 13-28) | Brief, moderate sessions may be considered with doctor approval | Major organogenesis complete; late-pregnancy cardiovascular strain hasn’t arrived yet |
| Third (weeks 29-40) | Most providers advise avoiding again | Enlarged uterus + higher baseline cardiovascular demand raise overheating/dehydration risk |
Who should avoid sauna completely, regardless of trimester
Certain conditions raise the risk enough that most guidance points to skipping sauna for the full pregnancy, not just adjusting timing: a high-risk pregnancy of any kind, a history of miscarriage, preeclampsia or gestational diabetes, placental abnormalities, a multiples pregnancy (twins or more), or any chronic cardiovascular condition. If any of these apply, that’s a conversation to have explicitly with your OB before any heat exposure — not a self-assessment call.
Warning signs to exit immediately
If sauna use has been cleared by a healthcare provider, stop a session right away for any of the following: dizziness or lightheadedness, nausea, unusual fatigue, a rapid or irregular heartbeat, shortness of breath, or simply feeling overheated. None of these are “push through it” symptoms during pregnancy.
The bottom line
- ACOG’s actual concern is a specific number — sustained core body temperature above 38.9°C (102°F) — not sauna use in general.
- The most-cited birth-defect study found sauna’s own risk wasn’t statistically significant, while hot tub exposure was (Milunsky et al., 1992) — a distinction most “sauna causes birth defects” content skips.
- A 2019 systematic review (Ravanelli et al.) found pregnant women’s thermoregulation kept core temperature under the danger threshold across the passive-heat protocols it reviewed, though it didn’t specifically test long, high-heat sauna sessions.
- First trimester carries the clearest risk because it covers neural tube closure; many providers consider brief second-trimester sessions lower-risk if approved.
- Infrared doesn’t get an automatic pass — cooler ambient air doesn’t guarantee a safe core temperature over a long enough session.
None of this replaces an actual conversation with your OB, who knows your specific health history and pregnancy. New to sauna in general and want the basics first? Sauna for beginners covers how to build a safe routine. Managing blood pressure alongside pregnancy? Sauna and blood pressure covers the cardiovascular side of heat exposure in more depth. Navigating a different hormonal life stage? Sauna for menopause covers how the guidance changes on the other side of pregnancy.
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