A pregnant person’s core body temperature typically sits between about 36.4 °C and 37.1 °C (97.5–98.8 °F), shifting gradually downward as pregnancy progresses. That range is slightly different from the textbook 37.0 °C often quoted for adults in general, and the direction of change surprises many people: rather than running hot for nine months, your core temperature actually drifts lower over time. The story behind that drift, and the thresholds where temperature becomes a genuine concern, is more nuanced than a single number on a thermometer can capture.
Why Pregnancy Shifts Your Baseline Temperature
The initial bump happens before most people even know they are pregnant. After ovulation, progesterone surges, and core temperature follows roughly 24 to 36 hours later.1Fertility and Sterility. Time relationships between basal body temperature and ovulation or plasma progestins If conception occurs, progesterone stays elevated instead of dropping before the next period. That sustained hormonal signal keeps body temperature slightly higher than the pre-ovulatory baseline, which is why a persistently elevated basal body temperature is one of the earliest indirect signs of pregnancy.
As the pregnancy continues, your body makes a series of cardiovascular adjustments. Blood volume expands, blood vessels near the skin relax, and sweating thresholds drop. All of these changes make it easier to dump heat into the environment. The result is that despite the extra metabolic work of growing a fetus and placenta, your core temperature does not keep climbing. Instead, these heat-dissipation mechanisms gradually pull it back down.2PubMed Central. Heat loss responses at rest and during exercise in pregnancy: A scoping review
How Temperature Changes Across Trimesters
A longitudinal study that tracked pregnant women from early pregnancy through the postpartum period measured core temperature at multiple points. At around eight weeks of gestation, average core temperature was about 37.1 °C (98.8 °F). From there it gradually declined toward term, and it kept falling after delivery, reaching its lowest point of about 36.4 °C (97.5 °F) at roughly 12 weeks postpartum.3PubMed. Core body temperature and the thermoneutral zone: a longitudinal study of normal human pregnancy So the warmest you are likely to feel, thermally speaking, is in the first trimester, and the coolest you may feel comes after delivery.
Wearable devices that continuously record skin temperature overnight are filling in finer detail. One study using wristband sensors found that nightly peak temperature rose noticeably after conception, climbing past the pre-pregnancy peak associated with ovulation. Over the following weeks and months, that peak slowly drifted down, eventually falling below the pre-pregnancy average by late pregnancy. Interestingly, the nightly temperature trough (the lowest overnight reading) moved in the opposite direction: it dipped at the start of pregnancy and then gradually rose all the way until delivery.4npj Digital Medicine. Biometrics of complete human pregnancy recorded by wearable devices The practical takeaway is that a single temperature reading can be misleading. Time of day and where you are in your pregnancy both shift what “normal” looks like for you.
Where You Take Your Temperature Matters
Not every thermometer gives you the same number, and those discrepancies matter more than usual when you are trying to decide whether a reading is concerning. Ear (tympanic) thermometers are fast and convenient, and one study in pregnant women found that tympanic readings did correlate with rectal readings. However, the correlation was not tight enough for the device’s estimate of rectal temperature to be considered clinically reliable.5Journal of Obstetric, Gynecologic & Neonatal Nursing. Tympanic Versus Rectal Thermometry in Pregnant Women In practice, that means an ear thermometer is fine for routine screening at home, but if your reading is borderline and you are worried, the number may be off by enough to matter.
Oral thermometers remain the most common home option and are generally adequate for spotting fevers. Forehead (temporal artery) scanners are popular too, but their accuracy can be influenced by sweating and skin blood flow, both of which change during pregnancy. Rectal readings remain the closest to true core temperature and are what most clinical studies use as a reference. If your healthcare provider asks for a specific number, it helps to mention how you took it.
When Does a Temperature Become a Fever?
In obstetric practice, a temperature of 38 °C (100.4 °F) or higher is generally treated as a fever.6PubMed Central. Fever in childbirth: a mini-review of epidural-related maternal fever That threshold applies during pregnancy and labor alike. Some clinicians distinguish mild fever (38–39 °C) from severe fever (above 39 °C), and the distinction is not just academic. Research on maternal fever during labor found a dose-dependent jump in complications: mild fever was linked to roughly a threefold increase in a composite measure of newborn problems, while severe fever was linked to about a sixfold increase.7American Journal of Obstetrics and Gynecology. Maternal fever in labor
A retrospective study of pregnant women who showed up to a gynecological emergency department with confirmed fever found that almost half were hospitalized, typically for about three days. Complications included early and late miscarriage, preterm delivery, severe sepsis, and in one case intensive-care admission.8PubMed Central. Causes and consequences of fever during pregnancy: A retrospective study in a gynaecological emergency department That does not mean every fever leads to disaster, but it does explain why clinicians take a pregnant person’s fever seriously and want to identify the cause quickly.
First-Trimester Fever and Birth Defects
The first trimester is when the developing embryo is most vulnerable to heat-related harm. A review of the evidence found that the effects of maternal fever appear to be both dose-dependent and timing-dependent, with the highest risk concentrated in those early weeks. Reported associations include neural tube defects, craniofacial abnormalities, congenital heart defects, neurodevelopmental disorders, fetal growth restriction, and pregnancy loss.9PubMed. Teratogen Update: Fever in Pregnancy
One large case-control study focused specifically on neural tube defects and found that mothers of affected babies reported fever around the time of conception about two and a half times as often as mothers of unaffected babies.10PubMed Central. Periconceptional maternal fever, folic acid intake, and the risk for neural tube defects Animal research adds some mechanistic detail: in guinea pigs, a core temperature spike of just 2–2.5 °C above baseline for one hour during early brain development increased the incidence of abnormally small brains.11PubMed. Effects of heat on embryos and foetuses No one can ethically run that experiment in humans, but the pattern across species and studies is consistent enough that the concern is well established.
A broader meta-analysis looking at neurodevelopmental disorders more generally found that maternal fever during pregnancy was associated with about a 24 percent increase in the odds of a neurodevelopmental condition in the child, with first-trimester fever specifically linked to a 13 percent increase.12PubMed Central. Fever during pregnancy as a risk factor for neurodevelopmental disorders: results from a systematic review and meta-analysis These are modest increases in relative risk, which means the absolute risk for any individual pregnancy remains small. Still, they are a good reason to bring a first-trimester fever to your provider’s attention rather than toughing it out.
Hot Tubs, Saunas, and the 39 °C Line
The concern over external heat sources during pregnancy comes from the same biological principle as fever: push core temperature high enough for long enough and you risk harming the embryo or fetus. A systematic review of the evidence concluded that the teratogenic risk threshold for core temperature is at or above 39.0 °C (102.2 °F).13PubMed. Heat stress and fetal risk. Environmental limits for exercise and passive heat stress during pregnancy: a systematic review with best evidence synthesis Animal data suggests the critical factor is a rise of about 2 °C above normal baseline, along with how long that elevation lasts.14PubMed. Risks of hyperthermia associated with hot tub or spa use by pregnant women
Does a normal hot-tub session actually push you there? An older but frequently cited study had women sit in tubs set at either 39.0 °C or 41.1 °C while monitoring their core temperature. None reached the 38.9 °C threshold before 15 minutes in the cooler tub or 10 minutes in the hotter one, and many got out from discomfort before hitting that mark at all.15PubMed Central. Suggested limits to the use of the hot tub and sauna by pregnant women So a quick dip is unlikely to be dangerous, but lingering in very hot water for an extended soak could become a problem. The standard advice to limit hot-tub sessions to around 10 minutes and avoid water above 40 °C is rooted in this data.
Exercise and Heat Regulation
Pregnant people often worry that exercising will raise core temperature into dangerous territory. The body’s enhanced heat-dissipation system during pregnancy actually works in your favor here. Increased blood flow to the skin, lower sweating thresholds, and greater fluid turnover all help cool you down. A study that put pregnant elite and recreational athletes through high-intensity interval running found that, while the pregnant participants did lose more fluid and showed larger increases in skin temperature than non-pregnant controls, they still managed the heat load.16PubMed Central. Cool mama: Temperature regulation during high-intensity interval running in pregnant elite and recreational athletes
The practical caution here is about environment rather than exercise per se. Working out in air-conditioned comfort is very different from running in midday heat during summer. Your body’s heat-dissipation system is impressive but not infinite, and humid conditions limit how well sweating can cool you. Staying hydrated, exercising during cooler parts of the day, and listening to your body’s cues (dizziness, nausea, feeling overheated) are more important during pregnancy than any specific heart-rate cutoff.
Extreme Heat and Working Conditions
With rising global temperatures, the relationship between environmental heat and pregnancy outcomes is getting more attention. A state-of-the-science review found that heat exposure can lead to a range of harmful outcomes for pregnant people and newborns, including hospitalization and death.17PubMed. The Impact of Extreme Heat Exposure on Pregnant People and Neonates: A State of the Science Review A prospective cohort study found that women exposed to high occupational heat had about double the risk of miscarriage and roughly double the risk of adverse outcomes at birth compared with those not exposed.18PubMed. Heat stress and adverse pregnancy outcome: Prospective cohort study
This is worth knowing if your job involves prolonged heat exposure, whether in kitchens, factories, agricultural fields, or outdoor construction. It also applies to anyone living through a heat wave without reliable air conditioning. The biological logic tracks with everything above: sustained elevation of core temperature stresses both the pregnant person’s cardiovascular system and the fetus. Keeping cool is not just about comfort. If you work in a hot environment, frequent breaks, shade, cold water, and cooling vests are genuinely protective steps, not luxuries.
The Fetus Runs Even Warmer
Something most people never think about is that the fetus inside the uterus is slightly warmer than the mother. Heat transfers to the fetus through the placenta and uterine wall, and fetal temperature typically sits about 0.3 to 0.5 °C above maternal core temperature. The umbilical circulation acts as the fetus’s main cooling system, carrying about 85 percent of fetal heat production back to the mother’s bloodstream for dissipation.19PubMed. Fetal and neonatal thermoregulation
This explains why the stakes of maternal hyperthermia are so much higher than they might seem from looking at adult temperature numbers alone. If you have a fever of 39.5 °C, the fetus may be sitting at 40 °C. That half-degree buffer matters during the critical windows of organ formation, particularly nervous-system development. It also means the fetus cannot cool itself independently. Everything depends on the mother’s ability to regulate her own temperature and maintain good placental blood flow.
Managing a Fever During Pregnancy
Acetaminophen (paracetamol) remains the go-to option for fever reduction during pregnancy. Globally, more than half of pregnant individuals use it at some point, mostly for pain, and it is considered the preferred antipyretic because of its long track record and the lack of safer alternatives.20ScienceDirect / American Journal of Obstetrics & Gynecology MFM. Safety of acetaminophen use in pregnancy: review of existing evidence Nonsteroidal anti-inflammatory drugs like ibuprofen are generally avoided, especially in the third trimester, because of effects on fetal circulation. Aspirin at analgesic doses carries similar concerns.
The goal is not to suppress every mild elevation but to prevent sustained high fevers. If your temperature reaches 38 °C, taking an appropriate dose of acetaminophen and contacting your provider is reasonable. If it reaches 39 °C or stays elevated despite treatment, that warrants more urgent evaluation. Most fevers in pregnancy have treatable causes: urinary tract infections, respiratory viruses, and other common infections. Early identification and treatment of the underlying cause, along with temperature control, is what protects both you and the fetus.
Low Temperatures and Hypothermia
While most of the attention goes to fevers and overheating, pregnant people can also be vulnerable to abnormally low temperatures, particularly around delivery. The same vascular changes that help dissipate heat during pregnancy, dilated blood vessels and expanded blood volume, also make it easier to lose heat rapidly in cool operating rooms or during blood loss. Add epidural anesthesia, which further dilates blood vessels and can suppress shivering, and the risk of hypothermia during cesarean delivery or prolonged labor is real. Obstetric patients are predisposed to hypothermia because of the combination of pregnancy-related vasodilation, anesthetic agents, and the blood loss and rapid fluid replacement that often accompany delivery.
This is mostly a concern for the clinical team managing your delivery rather than something you need to monitor at home during pregnancy. But if you feel unusually cold or shaky during or after labor, it is worth mentioning. Warming blankets and warmed intravenous fluids are standard measures in many labor units precisely because of this risk.
Postpartum Chills and the Temperature Reset
Many new parents are caught off guard by intense shivering or chills shortly after delivery. This phenomenon is strikingly common. One study found that about a third of patients experienced postpartum chills, which typically come on within minutes to an hour after birth and can be quite dramatic: teeth chattering, whole-body shaking, and a feeling of being freezing cold.21PubMed. Postpartum chills phenomenon: is it a feto-maternal transfusion reaction? The exact cause is not fully settled, but proposed explanations include small amounts of fetal blood entering the mother’s circulation, the rapid hormonal shifts after placenta delivery, and the sudden drop in metabolic heat production once the fetus and placenta are gone.
As noted earlier, core temperature continues to fall after delivery, reaching its lowest point around 12 weeks postpartum.3PubMed. Core body temperature and the thermoneutral zone: a longitudinal study of normal human pregnancy There is also evidence that women who breastfeed exclusively may see their core temperature stay elevated for at least six months postpartum compared with pre-pregnancy, likely because of the metabolic demands of milk production. The thermal landscape keeps shifting for months after delivery, so what feels like a normal temperature for you will continue to be a moving target well beyond the birth itself.