The short answer is that most medical guidance says avoid it, particularly in the first trimester, and that the decision belongs to you and your obstetrician rather than to any article.
What follows is the reasoning behind that guidance, because understanding why it exists is more useful than being told a rule. It also covers a distinction almost every source misses, which is that saunas and hot tubs are not the same risk.
What the concern actually is
The worry is maternal hyperthermia, meaning a rise in the mother’s core body temperature.
Human studies have found an association between high fever in early pregnancy and neural tube defects. Animal research points the same way and suggests the threshold that matters is roughly 2 degrees Celsius above normal baseline, with timing and duration mattering as much as the peak.
In practice that puts the number of concern somewhere around 39 degrees Celsius core temperature, and the window of concern in the first trimester, when the neural tube is forming.
Note what that is measuring. It is not the temperature of the room. It is the temperature inside the mother’s body, which is a different number entirely and much harder to reach than the room temperature suggests.
The distinction nobody makes
Four of the pages ranking for this question are titled some version of hot tub and sauna use, treating the two as one topic. There is good reason to think they are not.
In 1981, researchers measured actual core temperature in 20 women of childbearing age while they sat in hot tubs at 39.0 and 41.1 degrees Celsius, and in a sauna averaging 81.4 degrees.
In the cooler tub, five women reached the threshold temperature, though none in under 15 minutes. In the hotter tub, six reached it, none in under 10 minutes.
In the sauna, none of them did. Not one woman was able to stay in long enough for her core temperature to reach the level of concern. They left because they were uncomfortable first.
The physiology explains it. In a hot tub you are immersed in water hotter than your skin, sweat cannot evaporate, and there is no route to shed heat, so core temperature climbs steadily. In a sauna the air is dry, sweat evaporates, cooling works, and discomfort arrives well before core temperature does. Sauna temperature is high but the body has a functioning defense against it.
Why that does not mean a sauna is safe
This is the part where it would be easy to overreach, so it is worth being direct.
That study is from 1981, it included 20 people, and none of them were pregnant. Pregnancy changes thermoregulation, blood volume and cardiovascular response, so results in non-pregnant women do not transfer cleanly.
The finding also assumes a person leaves when uncomfortable. Someone determined to stay, or in a sauna hotter than 81 degrees, or ladling water onto the rocks, is in different territory.
And the consequence being avoided is severe enough that a conservative default is reasonable even on limited evidence. That is why the guidance is worded the way it is, and why the correct response to a study like this is a better understanding rather than a change in behavior.
Timing within the pregnancy
The first trimester carries the strongest warning because that is when the neural tube forms, roughly weeks three to four after conception. That window closes early, often before some people know they are pregnant.
After the first trimester the malformation concern falls away, but that does not make heat exposure neutral. The circulatory considerations are arguably larger later on. Blood volume peaks in the second and third trimesters, the cardiovascular system is already working harder, and blood pressure drops from heat land on top of that. Balance and mobility are also different late in pregnancy, which matters in a room with wooden benches, a wet floor and a step down on the way out.
So the risk does not disappear after twelve weeks. It changes shape, and the guidance stays cautious throughout for reasons that are no longer about the neural tube.
The risks people overlook
Neural tube defects dominate the discussion, and there are two more immediate issues that get less attention.
Fainting is the first. Blood pressure often runs lower in pregnancy, and heat lowers it further by dilating blood vessels. Standing up after a session is when people faint, and falling while pregnant carries its own consequences. Research in women who use saunas only occasionally found that syncope did occur, with nausea and confusion as the warning signs.
Dehydration is the second. Fluid needs are already higher in pregnancy, and how much fluid a session removes is a meaningful volume to lose when your blood volume is already doing more work than usual.
Neither of these has anything to do with the first trimester specifically. They apply throughout, and anyone with a condition on top of the pregnancy should know that those cautions stack.
Where the numbers come from
It is worth naming what the evidence base for this actually is, because the confidence in the guidance is higher than the confidence in the data.
There is no randomized trial on sauna use in pregnancy, and there never will be. Nobody is going to assign pregnant women to a heat exposure arm to see what happens.
So the guidance rests on three things: observational studies linking high fever in early pregnancy to neural tube defects, animal research establishing a temperature threshold, and a small number of measurement studies of how hot people actually get in tubs and saunas. Each of those is a step removed from the question being asked.
That is not a criticism of the guidance. When the evidence is indirect and the potential harm is a birth defect, erring toward caution is the correct professional response. It does mean anyone claiming certainty in either direction is going beyond what the literature supports.
The Finnish question
Someone will point out that Finnish women have used saunas throughout pregnancy for centuries, and that Finland does not have unusually high rates of the defects in question.
That observation is real and it is worth taking seriously. It is also not evidence in the sense that would change clinical guidance. Finnish sauna practice for pregnant women involves shorter sessions at lower temperatures on lower benches, in a population acclimated to heat from childhood, with cultural knowledge of when to leave that a first-time visitor does not have.
Cultural practice built over generations is not the same thing as a first-time user booking a session because she read that it was fine in Finland.
If you already used one
If you used a sauna before knowing you were pregnant, the 1981 measurements are genuinely reassuring: a normal session in a normal sauna is unlikely to have raised your core temperature to the level of concern.
Mention it to your obstetrician, who can put it in context with everything else about your pregnancy. Do not spend the week worrying about it on the strength of a web page, including this one.
What the guidance actually says
Major health services and pregnancy organizations recommend avoiding saunas and hot tubs during pregnancy, with the strongest wording around the first trimester. Some allow limited use with medical approval and specific limits.
The variation between sources is real, which is itself informative. It reflects a topic where the evidence is thin, the potential harm is serious, and the sensible institutional response is caution.
Your obstetrician knows your blood pressure, your history, and how your pregnancy is going. That context matters more than any general rule, and it is the reason this decision does not belong to an article.
Things that are not a substitute
A steam room is not a safer alternative. Saturated air blocks evaporation the same way water does, so the cooling defense that protects you in a dry sauna is absent.
A cold plunge paired with a sauna does not cancel the heat exposure, and cold water immersion carries its own cardiovascular considerations worth understanding separately.
An infrared cabin is not exempt. It runs cooler but the relevant question is core temperature, not air temperature.
What does work for the things people want from a sauna in pregnancy, meaning aches, sleep and stress, is a warm bath kept below body temperature, a prenatal massage, gentle movement, or simply rest. None of them carry a thermal question.
The bottom line
Most guidance says avoid a sauna while pregnant, especially in the first trimester, and that guidance is reasonable given what is at stake.
Saunas and hot tubs are not equivalent, and the measurement work that exists suggests a sauna is the less effective way to reach a concerning core temperature. That is context, not permission.
Fainting and dehydration are the risks that apply across the whole pregnancy and get the least attention.
Ask your obstetrician. There is a broader picture on sauna use for women that covers the rest.
