Heat during pregnancy: weeks 16–22, preterm birth research and safer summer habits in Japan
Understand a Japanese study of about 986,000 births, the heat association around gestational weeks 16–22, Tokyo–Osaka 2025 temperature tables and five practical ways to prevent heat illness.
Why extreme heat matters during pregnancy
Pregnancy changes blood circulation and the effort required to regulate body temperature. Hot, humid environments may make a pregnant person more vulnerable to dehydration, heat exhaustion and other heat-related illness. Heat can build up not only in the sun but inside poorly ventilated rooms or vehicles.
Protecting yourself from heat is a practical part of prenatal care, not an instruction to avoid normal activity altogether. Research has linked high-temperature exposure to adverse pregnancy outcomes, but it cannot establish that a single walk on a hot day will cause premature birth.
What did the new Japanese study investigate?
Researchers at Science Tokyo analyzed about 986,000 live singleton births recorded in Japan from 2016 through 2020. They paired perinatal records with regional weekly average temperatures, looking for pregnancy weeks when unusual heat exposure was most strongly associated with preterm birth.
For each prefecture, a high-temperature week meant a weekly mean above the local historical 90th percentile, roughly the highest 10 percent under the study definition. That is a location-relative research measure, not a universal Celsius threshold for everyone.
What do weeks 16–22 and an odds ratio of 1.16 mean?
The analysis found a statistically significant association between unusually hot weeks during gestational weeks 16–22 and higher odds of preterm delivery. The strongest estimate was at week 19: odds ratio 1.16, with a 95 percent confidence interval from 1.13 to 1.19.
An odds ratio of 1.16 describes roughly 16 percent higher odds in the compared groups. It does not mean an absolute increase of 16 percentage points or that 16 percent of exposed pregnancies will end prematurely. As an observational study, it cannot by itself establish causation.
| Study measure | Reported result | Correct interpretation |
|---|---|---|
| Sample | About 986,000 live singleton births, 2016–2020 | Nationwide observational data, not a causal experiment. |
| Exposure window | Pregnancy weeks 16–22 | Unusually hot weeks were linked with higher odds of preterm birth. |
| Strongest estimate: week 19 | OR 1.16 (95% CI 1.13–1.19) | Approximately 16% higher odds, not a 16-point absolute risk increase. |
Why might the middle of pregnancy be sensitive?
Between weeks 16 and 22, the fetus is growing rapidly and the cervix undergoes biological remodeling relevant to maintaining pregnancy. Researchers hypothesize that physiological stress from heat could influence inflammatory pathways or cervical changes, potentially contributing to an earlier onset of labor.
That mechanism remains a scientific hypothesis rather than something proven in individual patients. Calling the second trimester a more stable period does not make it risk-free, but the findings should not be interpreted as meaning every pregnancy at week 19 is in danger.
Heat precautions matter during every trimester
CDC guidance notes that pregnancy can increase vulnerability to heat illness and dehydration because the body works harder to cool itself. Other research has associated extreme temperatures in several stages of pregnancy with adverse outcomes, although the strength and mechanisms of those associations vary.
It therefore makes sense to avoid excessive heat throughout pregnancy rather than only in weeks 16–22. People with heart or kidney disease, pregnancy-related hypertension or jobs involving strenuous heat exposure should discuss an individualized heat-safety plan with their maternity team.
A research heat week is not the same as daily WBGT
The Japanese study compared average temperature for a week with historical local temperature distributions. For everyday decisions, Japan's heat alerts and wet-bulb globe temperature (WBGT) are more useful: WBGT accounts for humidity, radiant heat and air temperature instead of air temperature alone.
Under Japan's 2026 system, a heatstroke warning is issued when the forecast maximum WBGT reaches 33 or more at at least one observation point in the relevant warning area. A monthly or weekly research average cannot replace the current local warning or forecast.
Historical Tokyo and Osaka temperatures in 2025
Historical Japan Meteorological Agency data show particularly warm monthly average temperatures in Tokyo and Osaka during July through September 2025. A monthly mean includes daytime and nighttime readings across many days; it is neither the daily afternoon maximum nor the WBGT index.
The table illustrates what hot months looked like in those two cities, not a safe temperature limit for pregnant people or a prediction for the present year. Someone in Nagoya, Sapporo or elsewhere should consult conditions and alerts for their actual location.
| Month in 2025 | Tokyo average | Osaka average |
|---|---|---|
| May | 19.2°C | 20.1°C |
| June | 24.7°C | 25.4°C |
| July | 28.4°C | 30.2°C |
| August | 29.6°C | 30.8°C |
| September | 26.5°C | 28.1°C |
Five practical ways to reduce heat exposure
A reliable heat plan combines several ordinary measures: use air conditioning, drink fluids appropriately, avoid the hottest hours, wear breathable clothing and rest before exhaustion. No single cooling accessory can replace access to a cool environment when temperatures are dangerous.
Plan ahead with family or coworkers: identify drinking water, shade, air-conditioned stopping points and a way to change travel plans. The measures below reflect Japanese Ministry of Health and CDC advice, adapted to common needs during pregnancy.
| Measure | What to do | Important consideration |
|---|---|---|
| Air conditioning | Use cool indoor spaces and shade windows | There is no need to endure overheating. |
| Hydration | Sip regularly and carry drinking water | Follow medical limits on fluids and sodium when prescribed. |
| Outside plans | Choose cooler hours, hats and parasols | Check local WBGT warnings. |
| Clothing and cooling | Wear light fabrics and use a cool cloth | Accessories do not replace effective cooling. |
| Rest | Stop strenuous tasks before exhaustion | Seek help when symptoms persist or are severe. |
1. Use air conditioning and keep rooms comfortable
Air conditioning is a health protection tool in a heatwave, not something pregnant people should avoid because of fears that cool air will harm the abdomen. Use blinds or curtains, monitor actual indoor conditions and find an air-conditioned public place when home is excessively hot.
If your arms or feet feel chilly, a light extra layer can help while keeping the air comfortable. Fans improve air circulation but may not prevent overheating in extremely hot indoor spaces and should not be the only cooling option.
2. Drink regularly and consider individual fluid needs
Circulating blood volume and the body's cooling demands change in pregnancy, while sweating can increase fluid loss. Take regular opportunities to drink water, carry a bottle when traveling and be attentive to thirst and signs of dehydration. Plain water suits most ordinary situations.
Extended heavy sweating may call for replacing electrolytes, but salt tablets or sugary sports drinks are not automatically necessary. Anyone with heart or kidney disease or medical advice to limit fluid or sodium must follow their own clinician's instructions.
3. Choose cooler times for necessary trips
When possible, schedule walks, shopping or nonurgent errands for early morning or later evening rather than the hottest hours. Wide-brimmed hats, parasols, covered routes and breaks inside stations or air-conditioned shops help reduce direct heat exposure.
Check the local WBGT warning, trip duration and possible cooling stops before leaving home. Outdoor workers can discuss additional shaded rest breaks, access to drinking water and adjustments to strenuous tasks during dangerously hot conditions.
4. Dress comfortably and use cool packs safely
Lightweight and breathable clothing allows moisture to escape and can be more comfortable than tight, insulating outfits. A damp cloth or a cool pack wrapped in fabric may ease discomfort around the neck and other appropriate areas while resting.
Handheld fans, cooling scarves and umbrellas are useful additions but are not substitutes for fluids and a cool place. Avoid prolonged direct contact between very cold objects and skin, and stop exertion if you feel markedly weak, dizzy or nauseated.
5. Rest before reaching exhaustion
Standing on crowded transport, walking uphill, climbing stairs or doing household chores may become noticeably harder on hot days. There is no need to finish a plan at all costs because the second trimester is sometimes described as stable.
Build rest breaks into the day, ask trusted people to share demanding tasks and change plans when a heat alert is in effect. Stepping into a cool place before becoming unwell is a sensible preventive action, not a failure to cope.
Heat illness symptoms and when to call emergency services
Possible signs of heat-related illness include dizziness, headaches, muscle cramps, nausea, unusual weakness, intense thirst and passing very little urine. Stop activity, move to a cooler place, loosen tight clothing, cool the body and drink if fully alert and able to swallow safely.
Confusion, loss of consciousness, seizures or inability to drink independently require emergency care; in Japan, call 119. Never force water into someone who is unconscious. Persistent or worsening symptoms during pregnancy deserve prompt medical assessment.
Pregnancy warning signs that need obstetric advice
Do not assume every abdominal pain or tightening sensation is caused by the weather. Regular contractions before 37 weeks, significant abdominal pain, vaginal bleeding, possible fluid leakage or clearly reduced fetal movement once movement is regularly monitored should prompt immediate contact with a maternity service.
If symptoms include fainting, serious breathing difficulty, severe pain or another acute emergency, seek emergency care. People with a history of preterm birth or a high-risk pregnancy should follow the individualized plan provided by their obstetric team.
Prepare for a prolonged heatwave
Before trips, appointments or long commutes, review the weather and local heat alerts, identify shaded routes, water refill points and cooler rest locations, and keep contact information for your maternity service. Think through alternatives if the power goes out or the air conditioning fails.
Family members and colleagues can help reduce unnecessary physical demands and make schedule changes possible. This study offers a reason to take unusual heat seriously, but its population estimates cannot predict an individual baby's future; sensible cooling habits matter throughout pregnancy.
Sources: Science Tokyo 2026 observational study and Japan Meteorological Agency 2025 monthly observations. Odds ratio 1.16 is not a 16-percentage-point absolute risk increase; historical monthly temperatures are not today's WBGT or pregnancy safety cutoffs.