Heat exposure (hyperthermia) is considered a likely human teratogen – an environmental factor that can cause harmful effects to the fetus.
Hyperthermia causes a wide range of structural and functional defects, but there are many aspects to consider, and mechanisms by which hyperthermia causes these negative outcomes is not fully understood.
There is very little research specifically on the use of saunas, hot tubs, steam rooms, or hot showers and baths during pregnancy; most evidence pointing to potential harm comes from research in animals and non-pregnant individuals. This data is then applied to what is known about fetal development.
Although there is no official guidance, it is common for most HCPs to ask women to avoid hot tubs, saunas, showers and baths for a prolonged period of time (usually no more than 10 minutes).
Animal studies have shown that a rise in core body temperature of 3.0° to 4.5° F (1.7° to 2.5° C) or more was associated with adverse fetal outcomes; further, early embryos may be even more sensitive to heat damage at lower levels, regardless of the heat source.
Exposure to Heat
When the body is exposed to external sources of heat, such as hot climate, fever, electric blankets, hot tubs, saunas, steam rooms, or hot baths, the following occurs:
Under constant heat exposure, sweating starts to lose its effectiveness, therefore the body’s core temperature rises.
Pregnant women already have less of an ability to regulate their body temperature than non-pregnant women, based on the significant increase in blood volume early in pregnancy.
Additionally, the developing fetus has a limited capacity to regulate its own temperature.
Hyperthermia from external sources of heat is emerging as a risk factor for miscarriage, preterm birth, and a broad range of birth defects. It is possible that a significant rise in the body’s core temperature during pregnancy can affect fetal development based on the factors described above, especially in the first trimester.
A study published in October 2021 determined that potential windows of fetal vulnerability to heat appear to be weeks 3-9 (and 19-34) of pregnancy.
However, at least one study noted that although fetal heart rate increases during maternal hyperthermia, a healthy fetus appears to adjust well during and after heat exposure (likely depends upon length of exposure).
It is also possible that embryos and early fetuses may be more vulnerable to the effects of hyperthermia, and could experience effects from heat exposure before the mothers do.
Early embryos may also be more susceptible based on the stages of development that occur in early pregnancy: cell proliferation, cell migration, cell differentiation, and apoptosis (programmed cell death – helps create fingers and toes) are all necessary for proper organ development. Similarly, research regarding fevers has shown that a high prolonged fever results in cell death.
A study published in September 2021 assessed 9,588 infants with a congenital heart defect (CHD). The authors found that extreme heat events (EHE; 90 to 95th percentile daily ambient temperature) during 2-8 weeks after conception (4 to 10 weeks of pregnancy) was associated with overall CHD in offspring, particularly atrial septal defects and patent ductus arteriosus. The associations strengthened with the extent and cumulative days of maternal exposure to EHE.
Overall, research has also shown associations with cleft lip with or without cleft palate, gastroschisis, renal agenesis, limb reduction, and cardiac defects. Albeit inconsistent, the strongest evidence appears to be for neural tube defects, which have also been shown to occur with fever in early pregnancy.
A study published in November 2021 of more than 540,000 births found that heat exposure (in North Carolina, United States) was associated with an increased risk of preterm birth.
A study published in August 2021 used birth records for 315,226 infants born in Queensland, Australia, and matched to average maximum and minimum temperature for the last month of pregnancy. Higher temperatures were associated with small increases in the odds of delivering an infant in the extreme preterm period. This risk was further increased for women who smoked during pregnancy.
How hyperthermia leads to these effects is not known, but previous studies have suggested that variable factors may include the length/amount of heat exposure, stage of gestation, as well as the secretion of oxytocin and heat-shock proteins that result from heat stress.
Heat shock proteins play a crucial role in embryo-fetal development and are involved in every stage of the reproductive process. Lower numbers of heat shock proteins are associated with healthy pregnancies, and higher numbers are associated with various pregnancy-related complications; heat stress increases these proteins.
Heat exposure can also cause problems for pregnant women. A study published in July 2021 reviewed all emergency department (ED) visits and hospital admissions due to pregnancy complications from 2005 to 2013 in New York State. Daily mean temperature > 90th percentile of the monthly mean temperature in each county was defined as an Extreme Heat Exposure (EHE).
The study determined EHE was significantly associated with increased ED visits for pregnancy complications in summer. There was also a significant and stronger association in transitional months (May and September). The authors noted that "earlier warnings regarding extreme heat are recommended to decrease pregnancy complications."
Pregnant women are advised to avoid heat stress, defined as a core temperature of 102° F (39.0° C) or higher.
Weather: Exposure to hot weather can raise a pregnant woman’s core temperature and increase the risk for dehydration, heat exhaustion, and potentially even preterm birth (depending on exposure time).
Pregnant women are advised to avoid prolonged exposure to hot weather, remain in air conditioned rooms when possible, stay hydrated, and protect the skin with sunscreen, light clothing, and hats.
Exercise: Intense exercise during pregnancy has always presented a concern of hyperthermia. Exercise in hot conditions with inadequate fluid replacement is associated with reduced stroke volume and cardiac output, decreases in blood pressure, and reduced blood flow to muscle; it is theorized this could also lead to a decrease in blood flow to the uterus, fetus, and placenta as well.
Fortunately, of the limited studies conducted so far, there is no data suggesting that women exercise hard or long enough to cause significant hyperthermic effects during pregnancy. However, the risk for hyperthermia increases if women exercise in warm environments (i.e. no ventilation, high outdoor temperature, hot yoga, inadequate hydration). Read more.
Hot tub/whirlpool: Hot tub exposure appeared to have the strongest effect of any single heat exposure, but this type of exposure has not been well-studied in pregnant women. It has been shown, however, that a 104° F hot tub can raise a woman’s core body temperature to 102° F. Most hot tub water temperatures are in the range of 100° to 104° F.
Use of a hot tub or whirlpool bath after conception has been associated with a two-fold increased risk of miscarriage, which increased with frequency of use and exposure in early pregnancy.
Baths and showers: Several studies have indicated that any source of hot water can increase body temperature, which under the right circumstances, could impact fetal development.
Guidance from these studies indicates hot showers should be less than 15 minutes, but even 10-minute hot showers have shown to increase body temperature by at least 0.5° F. Women may be able to increase their bath time if water temperature is kept below "hot"; however, the relationship between length of shower/bath time, water temperature, and pregnancy-related outcomes is not known.
Previous studies have reported the average temperature of shower water in U.S. populations to be between 101° to 106° F (38.3° to 41.1° C) and typical bath water temperature to be between 93° to 113° F (33.8° to 45° C).
Note: A hot shower or bath of only a few minutes can drop a pregnant woman’s blood pressure, making her feel faint or weak upon standing up or getting out (see Exposure to Heat, above). Women should always ask for help getting out of a warm/hot bathtub, being careful not to slip and fall.
Put together, it is evident that prolonged exposure to heat sources can raise the body’s core temperature and potentially induce harmful effects on the fetus. However, “prolonged” exposure is not defined, and the minimum core body temperature at which damage may begin is not known.
Women should not focus on the source of heat exposure separately; any source of heat that can raise core body temperature during pregnancy for a prolonged period may lead to adverse pregnancy outcomes, depending on length and frequency of exposure.
Based on the research above, pregnant women are advised to:
Avoid prolonged exposure to hot weather
Remain in air conditioned rooms when possible
Protect the skin with sunscreen, light clothing, and hats
Avoid exercising in hot weather and hot environments (i.e. hot yoga)
Replace sweat lost with additional fluids, to include fluids with electrolytes if necessary
Keep shower and bath length (when using hot water) to 10 minutes or less (to be cautious)
Learn to recognize the signs and symptoms of heat stroke and exhaustion (listed above), in order to ask for help when necessary
Partners/Support can help pregnant women avoid hot climates, remain hydrated, keep cool, and seek medical care when necessary.
It is critical for partners, family members, and friends who live with or spend a significant amount of time with a pregnant women to recognize the signs and symptoms of heat stroke and exhaustion (see above).
According to the CDC, if a woman exhibits signs of heat stroke, partners/support are advised to call 911 right away, move her to a cool place and help lower her body temperature with cool cloths or a bath. It is also advised that the woman not be given anything to drink, as this may be especially unsafe if the individual is not alert.
If a woman exhibits signs of heat exhaustion, partners/support are advised to move the woman to a cool place, loosen her clothes, cool her body down with a cool cloth or bath, and have her sip water. However, if the woman begins vomiting, symptoms get worse, or if symptoms last longer than one hour, partners/support should get medical help right away.
Heat-Related Illnesses (U.S. Centers for Disease Control and Prevention)
Hyperthermia Fact Sheet (MotherToBaby.org)
Abnormal Development - Maternal Hyperthermia (UNSW Embryology)
Physical Activity and Exercise During Pregnancy and the Postpartum Period (includes heat exposure/stress) (American College of Obstetricians and Gynecologists)
Occupational (Employment) Exposure to Heat During Pregnancy (U.S. Centers for Disease Control and Prevention)