Are hot baths, saunas and hot tubs safe during pregnancy?

The verdict

Yes, within a time limit - and the time limit has actually been measured, which is unusual for pregnancy advice.

The threshold that matters is a maternal core temperature of 39.0C. Above that, in the first trimester, is where the neural tube defect signal sits. A 2019 systematic review in the British Journal of Sports Medicine pooled 12 studies covering 347 pregnant women and found that not one of them exceeded 39.0C. The highest core temperature recorded in any of them was 38.9C, and that was during exercise, not bathing.

Its conclusion is specific: pregnant women can sit in a 40C hot bath, or a 70C dry sauna at 15% humidity, for up to 20 minutes, at any stage of pregnancy, without reaching the teratogenic threshold. The mean end-of-session core temperature for hot water bathing across those studies was 36.9C - lower than for exercise.

Twenty minutes is the number. What is not safe is falling asleep in the bath, sitting in a hot tub for an hour, or doing repeated sauna rounds.

Why 39C, and why the first trimester

Hyperthermia is one of the few environmental exposures with a well-established human teratogenic mechanism. Raised core temperature during neurulation - the closure of the neural tube, which completes around 28 days after conception, about six weeks by the usual dating - is associated with an increased risk of neural tube defects including spina bifida and anencephaly. ACOG states it directly in its physical activity guidance: exposure to heat from sources such as hot tubs, saunas or fever has been associated with an increased risk of neural tube defects.

The 39.0C figure comes from animal teratology, extrapolated conservatively to humans; the supporting human epidemiology is observational and drawn largely from fever and hot tub use.

Two features of pregnancy make heat feel worse than it did before. Blood volume rises by around 40% and much of the extra flow goes to the skin, so you feel hot at a lower ambient temperature. And peripheral vasodilation lowers blood pressure, most markedly in the second trimester - which is why standing up out of a hot bath is the moment people faint.

The distinction that matters: water temperature is not body temperature

This is where most advice goes wrong. Sitting in 40C water does not make you 40C. Heat transfer is slow, your head and shoulders are out of the water radiating heat, you sweat, and you get out when you are uncomfortable. That is why measured core temperatures in the review were so much lower than the water - 36.9C after a hot bath.

A hot tub is the more demanding case because immersion is deeper and more prolonged, and because the water is thermostatically held rather than cooling over time as a bath does. Commercial units are typically set between 38 and 40C, and 40C (104F) is the conventional maximum.

How much is too much

• Hot bath: up to 40C, up to 20 minutes. A domestic bath cools continuously, so a bath run hot is below 40C within a few minutes anyway.
• Dry sauna: up to 70C at low humidity, up to 20 minutes, one session. Not repeated rounds with cold plunges in between - the review measured single exposures.
• Hot tub: the same 20-minute ceiling, applied more strictly, because the temperature is held. Sit on a higher seat so your chest and shoulders are out of the water if you can.
• Steam room: the least studied of the four. High humidity blocks evaporative cooling, which is the main route your body has for shedding heat, so the same air temperature is more thermally demanding. Treat it as the most conservative case.
• Labour pools: the one number a national body does publish. NICE's intrapartum care guideline requires that for women labouring in water, the temperature of the woman and the water be monitored hourly and that the water not be above 37.5C.

Get out earlier than the 20 minutes if you feel dizzy, lightheaded, sick, or your heart is pounding. Do not use any of them alone in the first trimester if you can avoid it, because fainting is the realistic hazard and it happens on the way out. Drink water before and after. Never combine with alcohol.

The other hot tub problem: what is in the water

Poorly maintained spa pools grow Legionella and Pseudomonas aeruginosa, and aerosolised spa water is a classic legionellosis exposure. Pregnancy reduces cell-mediated immunity, which raises the stakes on any infection. Favour a well-run pool over a lukewarm holiday-let hot tub nobody can date the last clean of.

If you have already had a long hot bath or sauna

Almost certainly nothing happened, and there is nothing to test. To exceed 39.0C you would have had to stay in far longer than is comfortable, in water hotter than domestic taps usually deliver, and you would have felt profoundly unwell doing it.

The scenario that actually warrants attention is not a bath at all: a fever. A temperature above 38C in the first trimester carries the same hyperthermia concern with none of the ability to step out of it, and a fever in pregnancy at any stage should be discussed with your midwife or GP - both for the temperature itself and because the infection causing it matters.

If you fainted, hit your head, or fell getting out, get that looked at on its own merits. If you have had any bleeding or abdominal pain since, that is a call to your maternity unit and it is not about the bath.

Where the guidance differs - and where the real disagreement is

Between countries, hardly at all. The genuine conflict here is between official guidance and the measured evidence.

The US gives a mechanism but not a number. ACOG's Committee Opinion 804 links hot tubs, saunas and fever to neural tube defects and tells pregnant women to stay well hydrated, wear loose clothing and avoid high heat and humidity, particularly in the first trimester. Its patient guidance names hot yoga and hot Pilates as activities to avoid because they may cause overheating. It does not publish a temperature or a duration. The CDC's clinical overview says heat exposure in any trimester has been associated with preterm birth, stillbirth and low birthweight, and that first-trimester exposure may increase the risk of certain birth defects - again with no bathing figure.

The UK has quietly gone silent. The NHS previously ran a dedicated page on saunas and jacuzzis in pregnancy; that page has been retired and now redirects elsewhere. The only live UK national number about warm-water immersion in pregnancy is NICE's 37.5C ceiling for labour pools, which exists to prevent maternal pyrexia during labour and is not a guideline for bathing at 12 weeks. Treating it as one would rule out an ordinary bath.

Canada advises caution without arithmetic. Provincial health services advise not letting the body get too hot in the first weeks and avoiding saunas and hot tubs then, while noting that experts do not forbid their use. Health Canada's published heat guidance treats pregnancy as a vulnerability factor for extreme heat generally rather than issuing a spa rule.

The 20-minute evidence is Australian, and Australia has not adopted it as guidance. The systematic review that produced the numbers came out of the University of Sydney. No Australian government body has turned it into consumer advice, and FSANZ and the national health department publish nothing on saunas in pregnancy.

So there is no country you can move to for a different answer. What there is, is a gap: every authority says avoid overheating, none of them says at what temperature or for how long, and the best available measurement says 20 minutes at 40C does not get you close.