At 39°C, Pregnancy Risks Rise 20%, a Review of 103 Studies Finds

Heatwaves are getting hotter and more frequent. For most of us, that means sticky nights and higher electricity bills.

For pregnant women, it may mean something more serious.

A new review in Nature Communications pulled together more than 100 studies on heat and pregnancy. It found the risk of problems such as premature birth starts rising on days above about 30°C (86°F).

And it keeps climbing as the thermometer goes up.

The researchers hope these numbers can become the basis for heat warnings designed specifically for pregnancy.

Why heat matters in pregnancy

Pregnancy changes how your body handles heat. Your body works harder, produces more heat, and needs more fluid.

Earlier research has linked hot weather to premature birth, low birth weight and stillbirth. Premature birth affects about 1 in 10 births worldwide, around 13.4 million babies a year.

But there's a practical gap.

Public health agencies need to know at what temperature to warn pregnant women. That's what this review set out to find.

The review: 103 studies, 42 million people

How it worked

Researchers led by Lund University in Sweden searched five scientific databases for studies on outdoor heat and pregnancy or birth outcomes.

They included 103 studies, covering nearly 42 million participants or records across six continents. Most were large cohort studies that followed pregnancies over time.

The outcomes included premature birth, low birth weight, stillbirth, birth defects, early rupture of the waters and high blood pressure in pregnancy.

Finding the thresholds

For the main analysis, they combined 21 studies with more than 5.5 million participants or records.

They looked at the day's maximum temperature and how risk changed on that same day. That's the kind of number a weather forecast gives you.

Why the day's high?

The team tested several ways of measuring heat. The daily maximum gave the clearest signal, and there were enough studies to analyse it properly.

Average daily temperature, by contrast, showed no clear link. It's also pulled down by cool nights, so it's less useful for spotting dangerous days.

The results: risk climbs with the heat

Here are the thresholds they estimated for any adverse pregnancy or birth outcome.

These are relative increases. A 10% rise means that if 7 in 100 pregnancies had a problem on a normal day, about 8 might on a very hot one.

The pattern held for premature birth on its own, with almost identical thresholds: about 30°C for a 5% rise and 38.6°C for a 20% rise.

Cooler places, lower thresholds

In temperate climates, like the UK or northern US, the thresholds were lower.

A 5% rise came at about 29°C (84°F), and a 20% rise at about 35.6°C (96°F). People in cooler places are less used to heat, so their bodies may cope less well.

What about high blood pressure in pregnancy?

The picture here was mixed. Some studies found higher risk of pre-eclampsia in the heat, and some found lower risk.

The authors suggest heat's natural tendency to lower blood pressure may explain part of this. Early rupture of the waters, by contrast, was more consistently linked to heat.

Individual studies show bigger swings

Some single studies in the review found larger jumps for premature birth. One found about 17% higher risk on days of 40°C or more compared with 20°C days.

Another, in Venice, found the risk nearly doubled on its hottest days compared with merely warm ones. Results like these vary a lot by place and method.

Heat warnings weren't built for pregnancy

Here's a problem the review highlights. More than 160 heat-stress indices exist, but most were designed around the bodies of young, healthy men.

Pregnant bodies respond differently. Yet only about 32% of the studies used combined measures that account for humidity and wind as well as temperature.

Maternal health itself is under-studied. Fewer than 20% of the studies looked at the mother's own health, rather than the baby's.

A three-level warning system

The authors propose warnings built on their three thresholds, at about 30.5°C, 34°C and 39°C.

They also stress that heat symptoms like dizziness, headaches and cramps start at lower temperatures than birth problems. So advice to stay cool should begin well before the first warning level.

Who is most at risk?

Heat doesn't hit everyone equally. The evidence points to a few groups who deserve extra care:

• Women on lower incomes, who may lack air conditioning or have less control over their working conditions.
• Very young and older mothers, where earlier research found the strongest links.
• People in cooler climates during heatwaves, since the thresholds there are lower.
• Women who work outdoors or in hot workplaces, whose exposure a daily forecast may underestimate.

That last point is my own inference, but it follows from the fact that the studies used outdoor weather, not personal exposure.

What earlier research found

Heat and premature birth

A 2020 review in The BMJ looked at 70 studies from 27 countries.

The odds of a premature birth rose about 5% for every 1°C increase in temperature, and about 16% during heatwaves. Stillbirths also rose about 5% per 1°C.

In 40 of 47 studies, premature births were more common at higher temperatures. All 8 stillbirth studies found a link with heat.

The links were strongest for women in poorer groups and at the youngest and oldest ages.

Beyond premature birth

A 2026 scoping review mapped 61 studies on heat and newborn health.

It found consistent links between heat and low birth weight, stillbirth, newborn deaths and birth defects. Risks were higher in socially and geographically vulnerable groups.

The body feels it first

A 2026 study in South Africa followed 201 pregnant and postpartum women with a mobile app.

Symptoms such as dizziness, thirst, exhaustion and poor sleep rose with heat. They appeared at fairly modest levels of heat stress.

The new review's authors make the same point. Warnings for comfort and symptoms need to kick in earlier than warnings about birth outcomes.

How it fits together

Put together, the evidence is consistent. Hotter days go with more pregnancy and birth problems, and the effect is sharper for vulnerable women.

What's new is turning that link into specific temperatures a warning system could use.

How much should you trust this?

Promising. The overall link is well supported, but the exact thresholds carry uncertainty.

What makes it convincing

• It draws on 103 studies and tens of millions of pregnancies or records.
• The protocol was registered in advance.
• It agrees with earlier reviews in The BMJ and elsewhere.
• The authors declare no competing interests.

What makes me cautious

• The studies are observational. They show links, not proof that heat causes each problem.
• Studies measured heat in many different ways, which made combining them hard.
• Only 21 studies could be used for the threshold calculations.
• One person carried out the quality assessment, and data weren't independently double-checked.
• Only four studies came from low-income countries, where heat risks may be greatest.
• Outdoor temperatures don't capture air conditioning, indoor heat or work conditions.

What this means for you

If you're pregnant, this doesn't mean you should panic on a hot day. The increases are modest, and most pregnancies go well.

But it's worth taking hot days seriously, especially above about 30°C (86°F):

• Stay cool. Spend the hottest hours in shade, air conditioning or a cool room.
• Drink regularly. Don't wait until you feel thirsty.
• Avoid heavy exertion in the middle of the day.
• Plan ahead. Check the forecast and have somewhere cool to go.
• Know the warning signs. Dizziness, headache, confusion, contractions or reduced baby movements need prompt medical advice.

If you're a partner, employer or neighbour, you can help too. Offering a cool space, flexible hours or a lift on a scorching day may matter more than it seems.

The NHS explains how to cope in hot weather. The World Health Organization also has a fact sheet on heat and health.

In this CBS Mornings segment, Dr Céline Gounder explains how extreme heat affects pregnancy:

What we still don't know

1. Which heat measure works best? Humidity and wind matter, but few studies used combined heat-stress indices.
2. Which weeks of pregnancy are most vulnerable? Evidence points to early and late pregnancy, but it isn't settled.
3. Does air conditioning cancel the risk? Home and work cooling weren't measured.
4. What about low-income countries? They're badly under-studied.
5. How does heat cause harm? The biological pathways remain poorly understood.
6. Do warnings actually help? Nobody has tested whether pregnancy-specific heat alerts improve outcomes.

My take: pregnant women need their own heat warnings

What I like about this review is how practical it is. It turns a large, messy body of research into numbers a weather service could use.

I'm cautious about the exact thresholds, because the underlying studies measured heat so differently.

I'd also like to see the thresholds tested in real warning systems. Do alerts at 30°C actually change behaviour, and do fewer babies arrive early as a result?

And the research gap in poorer, hotter countries worries me. The people facing the most heat are the ones we know least about.

Still, the direction is clear and consistent. As summers get hotter, pregnant women deserve heat warnings designed with them in mind.