Climate change poses a direct threat to human health, and growing evidence suggests pregnant people are especially vulnerable to hotter-than-normal summers.

A new analysis of 70 studies from around the world has found higher temperatures during pregnancy are linked to a small increase in preterm births and stillbirths, especially in low- and middle-income countries.

While the risk appears relatively minimal, scientists are worried it could have a major impact on public health in the future, especially with climate change driving more intense and frequent heat waves.

Just as young children, the elderly, and those with pre-existing conditions are warned about the dangers of extreme heat events, we should also warn those who are pregnant, they advise.

Carrying a child places many new demands on the human body, forcing the heart to work harder, raising internal temperatures, and leaving the body vulnerable to heat stress, exhaustion, and dehydration.

"Pregnant women merit a place alongside the groups typically considered as at high risk for heat related conditions," the international team concludes.

This field of research is still relatively new, but from what we know so far, there's reason to worry both for the mother and the baby.

None of the studies included in the current review are perfect or able to provide a clear cause or effect. Yet within the larger literature, the pattern is both consistent and concerning.

An observational study published last year based on an assessment of 56 million births in the US also identified a link between rising temperatures and shrinking gestation periods.

"When more and more studies start to pile up and coalesce around the same conclusion, we have to pay attention, especially when there's biological plausibility behind the outcome," explains obstetrician-gynecologist Nathaniel DeNicola in a separate 2019 paper on the subject.

Analysing more studies on the subject than ever before, the current meta-analysis examines how heat sensitivity impacts three outcomes in pregnancy: stillbirths, premature births, and low birth weight.

The research came from 24 countries, most of which were based in North America, the European Union, Australia, and New Zealand, although seven came from low- and middle-income countries.

For each 1°C increase in temperature, researchers found the risk of early birth and stillbirth increased by roughly 5 percent on average. In a prolonged heat wave, the risk of early birth rose by 16 percent.

To put that in perspective, the global average rate of preterm births is about 10 percent, so the impact of extreme heat, if there is one, is relatively small compared to all the other factors that can influence the outcomes of a pregnancy.

The analysis showed low birth weight, for example, occurred in only 3 percent of the infants born during a heatwave, and the relationship was found much less often.

While only 18 out of 28 studies found a link between birth weight and heat exposure, 40 out of 47 studies found a link between preterm births and heat exposure.

"The evidence was strongest and most consistent for heatwaves," the authors write, "although the largest effect sizes were from measures of the cumulative dose of heat over the whole of pregnancy."

This means heat exposure could very well add up throughout a pregnancy, although outcomes appear to fluctuate between certain socioeconomic groups.

For instance, while some of the studies suggest low- and middle-income pregnancies are vulnerable to heat exposure for the full nine months, other studies in high-income countries suggest the last weeks of pregnancy is where exposure is most risky.

The different methodologies used and the various different subpopulations examined makes it hard to generalise.

What's more, nearly a third of the studies included were deemed of low quality, which means the conclusions we can draw are limited.

Several studies, for instance, found preterm birth rates escalated only when temperatures exceeded 25°C (77°F), and this could explain why other research, which only included temperatures below this threshold, did not show similar results.

There are just so many factors to consider and control when it comes to pregnancy outcomes, including education, access to health care, food security, and availability of air conditioning. Even the sex of the fetus might play a role.

A study in Japan, for instance, found spontaneous abortions were higher among male fetuses after a period of heat exposure. Looking at all 70 studies, the new review found the same pattern.

What's driving these results is unclear. Some animal studies have found heat exposure during pregnancy can interfere with the synthesis of heat shock proteins, leading to fetal cell damage, oxidative stress or inflammation. Whether this holds up in humans remains to be seen.

Further research should be a high priority, especially since the pregnancy risks of heatwaves appear much higher in areas where far fewer protections exist.

"Exposure to high temperatures in agricultural and other outdoor work, could occur before the pregnancy is recognised, and, even late in pregnancy, poorer women might work beyond their heat tolerance limits to avoid losing pay," the authors worry.

From what we know so far, that's cause for concern, yet to date, many emergency heat plans around the world, including those in the United States and Europe, fail to include pregnant people as a vulnerable group.

"Pregnant women as an at-risk group to climate change are largely ignored," Skye Wheeler, an emergencies researcher for the Women's Rights Division of Human Rights Watch, told BuzzFeed News earlier this year.

That clearly needs to change.

The study was published in BMJ.