Heat can pose real risks during pregnancy, but the danger depends on how hot your body actually gets and how long it stays that way. The critical threshold is a core body temperature of about 102.2°F (39°C). Below that, your body and your baby are generally well protected. Above it, especially during early pregnancy, the risks to fetal development increase. Understanding the difference between a warm day and genuine overheating can help you stay safe without unnecessary worry.
Why Pregnancy Makes You More Heat-Sensitive
Your body is already running warmer than usual during pregnancy. Blood volume increases significantly, your metabolism speeds up, and you’re carrying extra weight. All of this generates more internal heat. At the same time, your body becomes less efficient at cooling itself through sweating, which means your core temperature can rise faster and stay elevated longer than it would otherwise. The CDC notes that pregnant people are more likely to develop heat exhaustion, heat stroke, or dehydration sooner than those who aren’t pregnant.
The Temperature That Matters Most
The concern isn’t really about the temperature outside. It’s about the temperature inside your body. Research on both animal models and human pregnancies has identified a core body temperature at or above 39°C (about 102.2°F) as the threshold where risk increases. That’s roughly 1.5 to 2 degrees above your normal baseline. Staying below that number, regardless of the heat source, keeps you in safe territory.
This threshold applies whether the heat comes from a fever, a long soak in a hot tub, intense exercise, or simply spending too much time outside on a scorching day. A systematic review of heat stress during pregnancy found that women can safely exercise for up to 35 minutes at high intensity in 77°F weather, sit in a hot bath (104°F) for up to 20 minutes, or exercise in water (up to about 92°F) for up to 45 minutes, all without their core temperature crossing the danger line. The key is duration and intensity, not just the temperature around you.
First Trimester: The Highest-Risk Window
The first 12 weeks of pregnancy are when heat exposure carries the greatest potential consequences. This is the period when the brain, spinal cord, and heart are forming, and elevated core temperature can interfere with that process. Several studies have found a small but real increased chance of neural tube defects (problems with brain and spinal cord development) when body temperature stays elevated before the sixth week of pregnancy. After week six, the risk of neural tube defects from heat drops considerably.
Other birth defects linked to sustained high body temperature in the first trimester include heart defects, abdominal wall defects, and cleft lip or palate. A large U.S. case-control study found that prolonged exposure to extreme heat events during early pregnancy was associated with increased odds of specific heart defects, particularly ventricular septal defects (small holes between the heart’s lower chambers). After 10 consecutive days of extreme heat in spring, the odds of this particular defect increased substantially. These associations were strongest in certain regions and during specific seasons, suggesting that cumulative heat exposure over multiple days matters more than a single hot afternoon.
Some studies also suggest a possible link between elevated body temperature and miscarriage, though this is harder to pin down because miscarriage has many potential causes, and separating the effect of heat from other factors is difficult.
Fever Versus Environmental Heat
Your body temperature can spike from two very different sources, and the risks differ slightly. Fever is an internal response, usually triggered by infection, and it can push your core temperature above the critical threshold quickly and keep it there for hours. Environmental heat or overheating (called hyperthermia) happens when your body absorbs more heat than it can release, whether from hot weather, exercise, saunas, or hot tubs.
Both can be harmful if your core temperature rises high enough for long enough. But fever in the first trimester has been more consistently linked to birth defects in research, partly because fevers tend to be sustained and because the underlying infection may contribute its own risks. With environmental heat, your body has more tools to regulate itself (moving to shade, drinking water, resting), so the exposure tends to be shorter. That said, untreated or prolonged overheating from any source carries similar concerns.
Heatwaves and Preterm Birth
Even later in pregnancy, when the risk of birth defects from heat drops, extreme outdoor temperatures can still affect outcomes. A study published in JAMA Network Open analyzed birth data from 50 U.S. metropolitan areas and found that heatwaves were associated with small but measurable increases in preterm and early-term births. After four consecutive days of temperatures above the local 97.5th percentile, preterm birth rates rose by about 2%. For younger mothers, those with less education, and racial or ethnic minorities, the increase was closer to 4%.
These are modest numbers at the individual level, but they reflect a real biological effect: sustained heat stress can trigger dehydration, reduce blood flow to the uterus, and increase levels of hormones associated with labor. If you’re in the third trimester during a heatwave, staying cool and hydrated is especially important.
How to Stay Safe in the Heat
The practical steps are straightforward. Drink at least 64 ounces of water daily, and increase that amount when you’re spending time in hot weather. Dehydration impairs your body’s ability to sweat and cool itself, which is already compromised during pregnancy. If your urine is dark or you feel thirsty, you’re already behind.
For exercise, the evidence is reassuring: moderate activity in reasonable conditions is safe. You don’t need to avoid working out. Just keep sessions under 35 minutes when it’s warm, stay below about 90% of your maximum heart rate, and choose cooler parts of the day. Swimming and water-based exercise are particularly good options because water helps dissipate heat. If you’re exercising in water, keep the water temperature at or below about 92°F.
Hot tubs and saunas aren’t off-limits the way many people assume, but the window is narrower than you might expect. Research shows you can safely sit in a 104°F bath for up to 20 minutes or a dry sauna (158°F, low humidity) for up to 20 minutes without exceeding the critical core temperature threshold. The safest approach is to keep these sessions short and get out if you start to feel overheated, dizzy, or nauseous.
On hot days, limit time outdoors during peak heat (typically 10 a.m. to 4 p.m.), wear lightweight and breathable clothing, and spend time in air-conditioned spaces when possible. Cool water on your wrists, neck, and forehead can help bring your temperature down quickly if you start feeling too warm.
Warning Signs to Watch For
Heat exhaustion can escalate quickly during pregnancy. Watch for heavy sweating that suddenly stops (a sign your cooling system is failing), dizziness, nausea, headache, rapid heartbeat, or muscle cramps. Confusion or an inability to think clearly signals possible heat stroke, which is a medical emergency. If you feel any of these symptoms, move to a cool environment, drink water, and apply cool cloths to your skin. Heat stroke requires immediate medical attention because core temperature can climb dangerously high within minutes.