REF-1403 · Ravanelli et al., 2019 · British Journal of Sports Medicine · Paper · PubMed

Is a sauna or a hot session OK while pregnant?

In 12 studies (347 women) no one reached 39 °C: up to 35 min hard at 25 °C, 45 min of water exercise, 20 min in a 70 °C sauna or 40 °C bath. Longer, hotter or humid: untested.

Main caveat: Read from the abstract only. Study sizes, designs, gestational ages, measurement sites for core temperature, risk of bias and the synthesis method could not be…
moderate evidenceMeasured on non-athletes and recreational people

Ravanelli and colleagues start from the advice that pregnant women avoid heat stress because maternal hyperthermia, defined as a core temperature of 39.0 °C or more, is associated with a risk of teratogenicity, and note that guidelines do not say which combinations of climate and activity to avoid and may therefore discourage physical activity unnecessarily.

They searched five databases to 12 July 2017 for studies reporting the core temperature of pregnant women, at any stage of gestation, during exercise or passive heat stress, and related core temperature to intensity, mode, ambient conditions and duration. Twelve studies with 347 women were included.

No woman exceeded 39.0 °C; the highest individual value was 38.9 °C, during land-based exercise. The highest mean end-trial core temperatures were 38.3 °C (95% CI 37.7-38.9) for land-based exercise, 37.5 °C (37.3-37.7) for water-immersion exercise, 36.9 °C (36.8-37.0) for hot-water bathing and 37.6 °C (37.5-37.7) for sauna.

The authors conclude that pregnant women can safely exercise for up to 35 min at 80-90% of maximum heart rate in 25 °C and 45% relative humidity, exercise in water of 33.4 °C or less for up to 45 min, and sit in hot baths (40 °C) or hot, dry saunas (70 °C, 15% relative humidity) for up to 20 min, irrespective of pregnancy stage, without exceeding the teratogenic threshold.

Our reading: these are the conditions that were studied, all of which stayed under 39.0 °C, so they are tested envelopes rather than limits; outside them the evidence is missing, not negative. The base's composite pregnancy entry advises "caution with hot-weather training and sauna" without a source (REF-339, C+); this review gives the tested exposures behind that caution.

Protocol

Design: systematic review with best-evidence synthesis.
Sources: EMBASE, MEDLINE, SCOPUS, CINAHL and Web of Science, from inception to 12 July 2017.
Eligibility: studies reporting the core temperature response of pregnant women, at any period of gestation, to exercise or passive heat stress.
Reference point: a maternal core temperature of 39.0 °C or more, which the authors call the proposed teratogenic threshold (taken from earlier literature, not tested here).
Characteristics analysed: intensity, mode, ambient conditions and duration of exercise or passive heat stress.
Included: n = 347 women in 12 studies; the size, design and gestational age of each study are not in the abstract.
Not in the abstract: how the best-evidence synthesis graded studies, risk of bias, how core temperature was measured, the trimesters represented, the women's fitness.
Funding and interests: PubMed prints "Competing interests: None declared."; funding is not available from the abstract or the PubMed record, and Crossref lists no funder. No device maker, federation or data platform is named.

Key Results

ExposureHighest mean end-trial core temperature (95% CI)Conditions the authors call safe (none exceeded 39.0 °C)
Land-based exercise38.3 °C (37.7-38.9); highest individual value 38.9 °Cup to 35 min at 80-90% of maximum heart rate in 25 °C, 45% relative humidity
Water-immersion exercise37.5 °C (37.3-37.7)water ≤33.4 °C, up to 45 min
Hot-water bathing36.9 °C (36.8-37.0)40 °C bath, up to 20 min
Sauna37.6 °C (37.5-37.7)hot, dry sauna at 70 °C, 15% relative humidity, up to 20 min
Margin: the highest mean core temperature straight after exercise (38.3 °C) was 0.7 °C below the proposed teratogenic threshold; the highest individual value (38.9 °C) was 0.1 °C below it.
Stage of pregnancy: the authors state the envelopes hold irrespective of pregnancy stage.
What was not studied (from what the abstract reports): heat or humidity above 25 °C and 45% for land exercise, efforts longer than 35 min at that intensity, races, repeated daily heat exposure, saunas or baths longer than 20 min or hotter than stated, and heat exposure straight after exercise.

Mechanisms

Measured: core temperature during and at the end of exercise or passive heat exposure in pregnant women; no value reached 39.0 °C.
Authors' interpretation: maternal hyperthermia of 39.0 °C or more is the risk to avoid; the studied combinations of intensity, mode, climate and duration stay below it, so guidelines need not discourage them.
Inference: core temperature rises with the rate of heat production (intensity), the time it is sustained and how hard the climate makes heat loss; the land-exercise envelope was studied in a mild, dry climate (25 °C, 45%), so hotter or more humid air, or a longer or harder effort, would be expected to push core temperature higher than seen here.A trained woman working at 80-90% of her maximum heart rate produces more heat in absolute terms than a less fit one, but also sweats more; which way that shifts the envelope is not known from these data.

Practical Application

Is a sauna or a hot session OK while pregnant? In 12 studies (347 women) no one reached 39 °C: up to 35 min hard at 25 °C, 45 min of water exercise, 20 min in a 70 °C sauna or 40 °C bath. Longer, hotter or humid: untested.
The envelopes are what was tested, not limits (B+, systematic review of small acute studies, from the abstract): every woman stayed under 39.0 °C within them. Beyond them there are no data, which is not the same as harm. The review measured core temperature only; whether a given temperature harms the fetus comes from earlier work on maternal hyperthermia that this review cites, not from this review.
Settles the unsourced sauna caution: REF-339 (C+, no resolvable source) says "caution with hot-weather training and sauna" and dropped its own 39 °C figure as unchecked; this review supplies a sourced use of the 39.0 °C threshold and shows that a 70 °C dry sauna or a 40 °C bath for up to 20 min, and moderate-to-hard exercise for up to 35 min in mild conditions, kept every woman below it. The caution still applies outside those conditions.
The base's heat guides now carry a pregnancy line: SYNTH-04 (heat acclimation with exercise in the heat, hot-water immersion and, as the guide's own practice, a 15-30-min sauna straight after training) and SYNTH-06 (altitude, heat and iron) did not mention pregnancy (checked in KB.md, 2026-10-28); since that day their eligibility lines exclude heat blocks, post-training sauna and altitude camps in pregnancy and point to SYNTH-95 (practice).A 15-30-min sauna straight after training goes beyond the longest sauna exposure studied here (20 min) and adds heat on top of exercise, which the abstract does not report as tested.Not from this paper; the base's own practice: the assistant does not prescribe heat-acclimation blocks, post-training sauna or hot-humid sessions in pregnancy, keeps any heat exposure inside the tested envelopes, and refers hot races and heat plans to the obstetric team.
Heart rate here is a description of the studied exercise (80-90% of maximum), not a heat rule; where a heart-rate limit in pregnancy comes from at all, see REF-1400 and REF-1401.
What it is good for as a source: the tested heat and exercise exposures in pregnancy and how far core temperature rose. What it does not show: fetal or birth outcomes; safety in hot-humid weather, long races, repeated or multi-day heat exposure, or heat straight after training; anything specific to athletes.

Limitations

Read from the abstract only. Study sizes, designs, gestational ages, measurement sites for core temperature, risk of bias and the synthesis method could not be checked.
A marker, not an outcome. Core temperature against a threshold from earlier literature; no fetal or birth outcomes.
Envelopes are the studied maxima. The conclusions describe conditions that were tested and stayed below 39.0 °C; they do not show that slightly hotter, longer or harder conditions would exceed it.
Mild climate for land exercise. 25 °C and 45% relative humidity; no hot-humid data in the abstract.
Small acute studies. 347 women across 12 studies, single exposures; no repeated-day or acclimation data.
Funding and interests: competing interests "None declared" (PubMed); funding not available; Crossref lists no funder.
How this entry was checked and changed
Why this level

(from the abstract) — a systematic review of small acute exposure studies with no control condition, not of trials (the rubric's A- is an SR/MA of trials), graded as the base grades reviews of observational studies (REF-727, REF-1395, REF-1402); the outcome is core temperature judged against a teratogenic threshold the review takes from earlier work and did not test, the "safe" envelopes are the hottest and hardest conditions that happened to be studied rather than limits found, and the synthesis method, risk of bias and the size of each study are not in the abstract. Read from the abstract (closed access; Europe PMC holds no full text, OpenAlex status closed, and the only free copy listed is on bjsm.bmj.com, which is behind a Cloudflare check and was not tried)