Most medical guidance advises pregnant people to avoid soaking in hot springs, especially during the first trimester, because raising your core body temperature above about 38.9°C (102°F) has been linked to an increased risk of certain birth defects. That said, the risk depends heavily on how hot the water actually is and how long you stay in it, and those two variables make the answer more nuanced than a blanket “no.”
Why Heat Is the Concern
The worry isn’t about hot water touching your skin. It’s about what happens inside your body when you soak long enough for your core temperature to climb. During the earliest weeks of pregnancy, the embryo’s neural tube is forming, and the cells involved in that process are sensitive to temperature. Elevated maternal body temperature can disrupt cell division, migration, and the normal programmed cell death that shapes developing organs, with effects that researchers have compared in severity to those of ionizing radiation.1PubMed. Effects of heat on embryos and foetuses
The evidence linking maternal heat exposure to birth defects goes back decades. A well-known study published in JAMA found that women who used hot tubs in the first trimester had roughly triple the risk of having a baby with a neural tube defect compared to women who avoided heat exposure. When researchers adjusted for other factors like maternal age and folic acid use, the risk for hot tub users remained about 2.8 times higher. Hot tub use appeared to carry a stronger individual effect than sauna use or fever alone, and the risk escalated when women experienced more than one type of heat exposure.2JAMA. Maternal Heat Exposure and Neural Tube Defects
A later study examining 17 different birth defects found that mothers who reported any hot tub use in early pregnancy were significantly more likely to have infants with gastroschisis, an abdominal wall defect, and anencephaly, a severe brain malformation.3PubMed. Maternal use of hot tub and major structural birth defects These aren’t enormous absolute risks for any one person, but the pattern across multiple studies is consistent enough that public health authorities have taken it seriously.
How Long Before Your Core Temperature Gets Too High
The threshold most researchers and clinicians use is a core body temperature of 38.9°C (about 102°F). The question then becomes: how quickly does soaking in hot water push you to that point? The answer varies quite a bit depending on water temperature.
A study that measured vaginal temperatures of women sitting in hot tubs found that in water set to 39°C (about 102°F), none of the participants reached the 38.9°C core threshold before 15 minutes of soaking. In hotter water at 41.1°C (about 106°F), no one reached it before 10 minutes. And many women in both groups left before hitting that threshold because they simply became too uncomfortable to stay.4PubMed Central. Suggested limits to the use of the hot tub and sauna by pregnant women The researchers concluded that typical, brief hot tub use is unlikely to raise core temperature to dangerous levels, though prolonged soaking could.
A more recent systematic review synthesized the best available evidence and concluded that pregnant women can sit in hot baths at 40°C (104°F) for up to 20 minutes without their core temperature exceeding the threshold associated with birth defects, regardless of the stage of pregnancy.5PubMed. Heat stress and fetal risk. Environmental limits for exercise and passive heat stress during pregnancy: a systematic review with best evidence synthesis That finding suggests the risk is more about duration and water temperature than about any brief contact with warm water.
These numbers matter because they push back against the idea that any contact with hot water is dangerous during pregnancy. A quick dip in a warm pool is very different from a 45-minute soak in scalding water. The science consistently points to a combination of high temperature and extended time as the actual problem.
How Natural Hot Springs Differ from Hot Tubs
Most of the studies on heat exposure during pregnancy were conducted using commercial hot tubs, which are typically regulated to a specific temperature, usually around 40°C (104°F) in many countries. Natural hot springs are a different animal. Their temperatures can range wildly, from barely warm at around 25°C (77°F) to well above 45°C (113°F), depending on the geological source. Some hot springs that people actually bathe in hover around 37-40°C, while others are considerably hotter.
This variability is the core problem with applying hot tub research directly to natural hot springs. In a commercial hot tub, you can check the thermostat. In a natural spring, the temperature may shift from one spot to another, change with the seasons, or be hotter at the source and cooler at the edges. You may have no reliable way to know how hot the water actually is unless the site posts a measured temperature, and even then conditions can fluctuate.
If you find yourself at a natural hot spring that’s been measured at around 37-38°C and you limit your time, the thermal risk profile resembles that of a warm bath at home. But if the spring runs hotter than 40°C, or if there’s no way to verify the temperature, the conservative approach is to stay out, particularly in the first trimester when the neural tube is most vulnerable to heat disruption.
Infection Risks in Natural Springs
Temperature isn’t the only concern with natural hot springs. Unlike chlorinated pools or well-maintained hot tubs, natural springs sit in open environments where the water chemistry depends on local geology and whatever organisms happen to thrive there. Warm, mineral-rich water can be a hospitable environment for bacteria.
A study of a recreational hot spring in Uganda, for example, found that 30% of water samples contained Klebsiella pneumoniae, and 100% of those isolates were resistant to multiple antibiotics including ampicillin and ceftazidime. The researchers described the spring as a “hot spot” for the spread of drug-resistant bacteria.6Kampala International University. Monitoring Multi-Drug Resistant Klebsiella pneumoniae in Kitagata Hot Spring, Sheema District Southwestern Uganda: A Public Health ImplicatioN That’s one spring in one country, and conditions vary enormously across hot springs worldwide. But it illustrates the point that “natural” doesn’t mean “clean.”
During pregnancy, your immune system undergoes adjustments that can make you more susceptible to certain infections. Organisms that a non-pregnant person’s body might fend off easily, like certain waterborne bacteria or amoebas, can pose a more serious threat. Natural springs can also harbor organisms like Naegleria fowleri in certain climates, though this is rare and usually associated with freshwater lakes and rivers rather than hot springs specifically. The broader point is that the infection risk in an unregulated natural water source adds a layer of concern beyond heat alone.
What About the Minerals
Hot springs are often marketed for their mineral content. Sulfur, magnesium, calcium, and various trace elements dissolve into the water from surrounding rock. Some people find these minerals soothing for skin conditions, and there’s a long tradition of balneotherapy (therapeutic bathing) built around these claims. During pregnancy, you might wonder whether absorbing these minerals through your skin is helpful, harmful, or irrelevant.
In practical terms, the amount of any mineral your body absorbs through skin during a brief soak is very small. Your skin is a reasonably effective barrier, and passive soaking doesn’t deliver minerals into your bloodstream at anywhere near the rate that eating or drinking them would. The rare exception involves springs with unusually high concentrations of heavy metals like arsenic or mercury, which can occur naturally depending on the geology. Springs used for recreational bathing in developed countries are typically tested for these, but backcountry or unregulated springs may not be.
For most developed-world hot spring resorts, the mineral content is unlikely to pose a meaningful risk during pregnancy. The heat remains the more important variable by a wide margin.
Chemical Disinfectants in Managed Springs
Some commercial hot spring facilities treat their water with chlorine or bromine, just like a swimming pool. If you’re pregnant and weighing the risks, you might wonder whether those chemicals add a separate concern. A study of pregnant women who swam regularly in chlorinated pools found a small reduction in one measure of head size related to how long they spent in the water, though no association was found with birth weight or birth length.7PubMed Central. Maternal swimming pool exposure during pregnancy in relation to birth outcomes and cord blood DNA methylation among private well users The effects were subtle, and swimming pools involve continuous physical activity that hot spring soaking doesn’t, so the comparison has limits.
In general, the chlorine or bromine levels used in commercial facilities are considered safe for pregnant swimmers and bathers at normal exposure durations. The disinfection byproducts that form when chlorine reacts with organic matter are a topic of ongoing research, but the evidence doesn’t point to a clear risk at the levels found in well-maintained facilities. If anything, a properly disinfected hot spring facility may be safer from an infection standpoint than an untreated natural one, even if it feels less romantic.
What If You Already Soaked Before Knowing You Were Pregnant
This is one of the most common worries, and it deserves a straightforward answer: a single session in a hot spring or hot tub is very unlikely to have caused harm. The research on heat-related birth defects involves patterns of repeated or prolonged exposure. The increased risks found in the studies cited above are population-level averages across groups of women, many of whom reported regular use rather than one-time exposure.
Researchers who have reviewed the evidence have recommended that women who are or may be pregnant be advised of the standard limits, but also that they be empowered to monitor water temperature and keep their body temperature below the 38.9°C threshold rather than simply told to avoid warm water entirely.8PubMed. Risks of hyperthermia associated with hot tub or spa use by pregnant women The tone of the clinical literature is cautionary, not alarmist. If you spent 15 or 20 minutes in a hot spring before you realized you were pregnant, your absolute risk of a problem remains very low.
That said, if you know you’re pregnant and plan to visit hot springs, the evidence supports a few practical limits: keep the water at or below 40°C if you can verify it, limit your soak to under 20 minutes, and get out if you start feeling overheated, dizzy, or flushed. Your body’s own discomfort signals are actually a useful safety mechanism here. As the threshold studies showed, many women leave hot water before reaching dangerous temperatures simply because they feel too warm.
Trimester Matters
The first trimester is the period of highest concern. Neural tube closure happens within the first four weeks after conception, often before a person even knows they are pregnant. The studies showing the strongest associations between heat exposure and birth defects focused specifically on early pregnancy, when the fundamental architecture of the embryo’s nervous system and organs is being established.2JAMA. Maternal Heat Exposure and Neural Tube Defects
By the second and third trimesters, the major organs have formed, and the primary risks shift. Prolonged heat exposure later in pregnancy is more associated with cardiovascular strain on the mother, including drops in blood pressure, dizziness, and dehydration, rather than structural birth defects. Your blood volume increases substantially during pregnancy, and the way your body regulates temperature changes too. Hot water can cause blood vessels to dilate, which may temporarily lower blood pressure and leave you feeling lightheaded. In a slippery hot spring environment, that’s a practical safety concern even if the embryological risk has decreased.
The systematic review that found 20 minutes in 40°C water to be safe noted that this held “irrespective of pregnancy stage,” meaning the thermal safety margin applied across all trimesters.5PubMed. Heat stress and fetal risk. Environmental limits for exercise and passive heat stress during pregnancy: a systematic review with best evidence synthesis So the 20-minute guideline appears to be a reasonable general limit regardless of when in pregnancy you’re considering a soak.
Japan’s Evolving Guidance on Pregnancy and Hot Springs
Japan has one of the world’s deepest cultural relationships with hot spring bathing, or onsen. For decades, Japanese bathing facility guidelines actually listed pregnancy as a contraindication for hot spring use. This sparked debate among researchers and physicians, partly because the restriction was so sweeping that it didn’t distinguish between mildly warm and very hot springs, and partly because millions of Japanese women had bathed in onsen during pregnancy for centuries without any clear epidemic of heat-related birth defects.
In 2014, Japan’s Ministry of the Environment revised its hot spring guidelines and removed pregnancy from the list of contraindications. The change reflected a growing body of evidence, including domestic research, suggesting that moderate hot spring use didn’t pose the kind of categorical danger the older guidelines implied. The revised approach acknowledged that temperature and duration matter more than the act of entering a hot spring itself.
This cultural shift is worth knowing about because it mirrors the broader trajectory of the research: the conversation has moved from “hot water is dangerous during pregnancy” to “extreme heat exposure for extended periods is risky, and here are the limits that appear to be safe.” Japan’s experience suggests that a society deeply invested in hot spring bathing found a way to reconcile the practice with the evidence rather than abandoning it entirely.
Practical Decisions at the Spring
If you’re pregnant and standing at the edge of a hot spring trying to decide whether to get in, the key variables are temperature, time, and your own body’s signals. Here’s what the research supports:
- Check the temperature: If the facility posts water temperatures or you can measure the water yourself, look for 40°C (104°F) or below. Many developed hot spring resorts have pools at varying temperatures, and cooler pools may be well within the safe range.
- Limit your time: Even in water at the upper end of the comfortable range, staying under 20 minutes keeps your core temperature below the threshold associated with birth defects based on the best available evidence.
- Keep your upper body out: Sitting on a ledge with your legs in the water or soaking only up to your waist reduces how quickly your core temperature rises compared to full immersion up to your neck.
- Listen to discomfort: Feeling overheated, flushed, or dizzy is your body telling you it’s working hard to cool down. Get out. Research shows many women leave hot water before reaching dangerous temperatures because of discomfort alone.
- Stay hydrated: Hot water causes sweating even when you’re submerged, and dehydration can compound the cardiovascular effects of heat.
- Avoid unregulated backcountry springs: Without temperature data or water quality testing, you’re guessing at both the thermal and microbial risk.
None of these strategies eliminate every possible risk, but they address the specific mechanisms that the research has identified as problematic. A brief soak in a warm, clean, well-managed hot spring with a verified temperature is a fundamentally different exposure than a long soak in an untested, extremely hot natural pool. The evidence supports treating them differently rather than applying the same blanket warning to both.