Heat and Pregnancy

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Extreme heat is now one of the most serious environmental threats to a healthy pregnancy. Because of where many of us live, work, and give birth, pregnant Black people are exposed to more of it and harmed by it more often and more severely.The Centers for Disease Control and Prevention (CDC) states that the essence of preconception health involves getting and staying healthy overall.

What does extreme heat have to do with pregnancy?

An article by Rebecca Lindsey reports that the year 2024 was the hottest ever recorded, and the heat does not seem to be letting up. Since 1850, when global temperature tracking was implemented, we have seen the hottest years in the last decade. The record holder for hottest year before 2024 was 2023, and the years 2015 through 2025 contain all 10 of the hottest years on record. When the temperature climbs, a pregnant body has to work harder than usual to stay cool, and that extra strain has been linked to stillbirth, early labor, low birth weight, and other maternal health complications. The Centers for Disease Control and Prevention (CDC) now lists pregnant women among the groups at higher risk on hot days, alongside older adults, young children, and people with heart or lung conditions. 

How does heat affect a pregnant body?

A pregnant body is already working for two, and cooling down is part of that work. The CDC explains that heat raises the risk of pregnancy complications for several connected reasons:

  • Your body has to cool two people, not one. This means heat exhaustion and heat stroke can come on faster during pregnancy than they would otherwise.
  • Pregnancy makes dehydration more likely. A pregnant person requires more water, raising the risk of dehydration under normal circumstances. Additional heat causes the body to sweat because it is the main way the body sheds heat, and this increases the risk of dehydration. A dehydrated body cannot sweat as efficiently so it may not be able to keep a steady, healthy temperature. In this way, dehydration and overheating feed into each other.
  • A rising core temperature can harm fetal development. Elevated internal body temperature and fever during pregnancy have been linked to birth defects and other complications, especially early in pregnancy.

Later in pregnancy, the danger shifts toward the timing of the birth itself, which is where the research becomes especially clear.

What does the research show?

The effects of heat on pregnancy are no longer a matter of speculation. Two major reviews of global evidence have reached consistent conclusions:

  • In 2020, a review by Dr. Chersich, PhD, pooling 70 studies across 27 countries, found that the risk of preterm birth and stillbirth rose about 5% for every 1.8°F increase in temperature. Preterm birth rose to about 16% during heatwaves. 
  • Drawing on nearly 200 studies across 66 countries, Dr. Lakhoo MD., confirmed the higher odds of preterm birth during heatwaves and added further harms, including raised risks of birth defects and gestational diabetes, a form of high blood sugar that develops during pregnancy in someone who didn't have diabetes before. This occurs when pregnancy hormones interfere with how the body uses insulin; it usually resolves after delivery but raises the risk of complications during pregnancy and of having type 2 diabetes later on in life.

Each percentage may sound small on its own. But heat exposure is now nearly universal, and preterm birth and stillbirth are among the most consequential outcomes in public health. Both reviews found that the harm from heat was greatest among pregnant people, lower-income groups, and those at the extremes of age, including newborns and the elderly.

What are the warning signs of heat-related illness?

If your body gets too hot, you can become ill quickly. Learn these signs, and seek medical care right away if they appear during pregnancy:

  • Unusually heavy sweating
  • Headache
  • Muscle cramping
  • Dizziness or weakness
  • Nausea
  • Shortness of breath
  • Unusual tiredness

Because some of these can also signal a pregnancy complication, do not wait it out. When in doubt, call your healthcare provider or seek care.

How can you protect yourself on hot days?

Most of the CDC's guidance for pregnant people costs little or nothing. Think of it as a plan you build before the heat arrives, ideally with your healthcare provider or midwife.

  • Know your risk before you go outside. The CDC's HeatRisk Dashboard lets you enter your zip code for a color-coded forecast (yellow, orange, red, or magenta). Many pregnant people feel the effects on orange days, and some even on yellow ones. Ask your provider which color should be your signal to change plans.
  • Stay cool. If going outdoors, stay in the shade, take frequent breaks, and save activity for the cooler morning or evening hours. When staying indoors, use air conditioning if you have it–and if you don't, find somewhere that does such as a library, mall, or public cooling center (in many communities you can dial 211 to find one). A fan helps only when indoor temperatures are below about 90°F; above that, a fan can actually raise your body temperature.
  • Stay hydrated. Carry a water bottle and refill it throughout the day. Limit drinks that are high in sugar, salt, caffeine, or alcohol. If your urine is light yellow or clear, you are probably drinking enough. Because heavy sweating strips electrolytes such as salt and other minerals, ask your provider how to replace them safely during pregnancy.
  • Don't go it alone. Decide in advance who can check on you during a heatwave, and offer to do the same for a pregnant neighbor, an older relative, or anyone living alone. On the most dangerous days, that phone call can be a real form of protection.

What about air quality and your medications?

Heat often makes air pollution worse, and poor air quality can worsen pregnancy symptoms. The HeatRisk Dashboard also shows your local Air Quality Index (AQI), which runs from 0 (good) to 500 (hazardous). On days above 100, consider limiting outdoor activity; indoors, you can run a portable air purifier if you have access to one and reduce sources of indoor smoke like burning candles. Some pregnant people are sensitive even in the 51–100 range, so ask your provider whether that applies to you.

Have a plan for your medicines. Some medications increase the risk of dehydration or overheating, and some must be stored out of the heat, but never stop or change a medication without talking to your provider first. Because heat can cause power outages, make a backup plan for any refrigerated medicines or electronic medical devices.

Before the next heatwave, work with your provider to answer a few questions in advance: 

  • Which HeatRisk color is your signal to take action? 
  • Where can you go to cool down if your home gets too hot? 
  • Who will check on you on dangerous days? 
  • Do any of your medicines need special handling in the heat? 
  • Writing the answers down now means you won't have to figure them out in the middle of a heatwave.

Why are Black women more at risk?

Where you live changes how much heat you face. Neighborhoods with more asphalt and fewer trees trap heat and stay warmer long after sunset: the "urban heat island" effect. In American cities this often means that Black and Brown communities face the brunt of the heat. Combine that with older housing, limited or unaffordable air conditioning, and tighter budgets, and many Black pregnant people have fewer opportunities to escape when temperatures spike. The hottest neighborhoods are often the ones shaped by redlining and decades of disinvestment. These are disproportionately the neighborhoods where Black families live. To learn more about the connection between our environment and reproductive health, visit RSP's page on Environmental Justice and Reproductive Health by clicking here. For more about the history of redlining and food access, click here.

Black women in the United States are already about 1.6 times as likely as white women to deliver prematurely, and rising temperatures are only worsening this crisis. The birth data reflect this. A study by Dr. Avalos, PhD., found that a weekly increase of over 10°F during warmer months correlated with an 11.6% increase in spontaneous preterm birth. That increase was sharpest in communities of color, with Black women seeing a notably larger jump than white women. Other research found that Black women were significantly more likely to go into premature labor after heatwave exposure.

These gaps cannot be explained by income or education alone. Even when they have a college degree, private insurance, and early prenatal care, Black women remain about 1.6 times as likely to give birth prematurely. A multidisciplinary group convened by the March of Dimes reviewed more than two dozen suspected causes and concluded that racism itself best explains the disparity. Researchers describe a process sometimes called "weathering," the cumulative biological toll of chronic stress and discrimination, which wears on the same stress-response and immune systems involved in triggering labor.

Several biological pathways connect chronic stress to early birth, and they run through the immune system. Even a healthy, full-term labor begins as an inflammatory event: in the days before delivery, immune cells move into the uterus, cervix, and fetal membranes and release signaling molecules called cytokines, along with specific, specialized hormones, which soften the cervix and switch the uterine muscle from quiet to contracting. That is why chronic stress may matter so much here. Living with discrimination and sustained adversity appears to shift the immune system toward a persistently inflamed baseline. The body's natural anti-inflammatory brake, cortisol, becomes less effective over time, and pro-inflammatory activity runs less checked. Dr. Braveman, MD’s research found that stress hormones also prompt the placenta to release more stress-related hormones, which can make a pregnant person more susceptible to earlier delivery. If a pregnant person’s inflammatory baseline is already elevated, a small additional push, whether an infection or extreme heat, could be enough to tip the system into labor. 

A brief history of climate change

Systemic and Collective Change

While there are personal actions you can take to protect yourself, the deeper fix is collective: cooler, greener, better-resourced neighborhoods, livable housing, and a maternal health system that takes Black women seriously when they have concerns. Knowing what the heat is doing to your body, and refusing to accept that some mothers should simply absorb more of it than others, is where that work begins.

For more information about heat, pregnancy, and your health, please contact the following organizations: