Exposure to high temperatures linked to poor pregnancy outcomes

Although the effects may appear small in size, the researchers say the findings "could have a major impact on public health as exposure to high temperatures is common and escalating." Moreover, conditions such as preterm birth often have lifelong implications for affected newborns.

Increases in global temperatures raise concerns about heat impacts on health, especially in vulnerable groups such as the elderly, those living in poverty, and the chronically ill.

Previous evidence reviews have found associations between exposure to high temperatures and pregnancy outcomes, but have included few studies and have not assessed differences across population groups and by type of heat exposure.

So an international team of researchers set out to assess whether exposure to high temperatures in pregnancy is associated with increased risk for preterm birth, low birth weight and stillbirth.

They analysed the results of 70 studies, set in 27 countries, seven of which were low and middle income countries, that reported associations between high temperatures and preterm birth, birth weight and stillbirths.

Differences in study design and quality of evidence were taken into account.

WHO estimates that 15 million babies are born preterm each year, the leading cause of death among children under 5. The average rate of preterm birth in the studies was 5.6%, much lower than the global average of about 10%.

Of the 47 studies that assessed preterm births, 40 reported that preterm births were more common at higher than lower temperatures. Further analysis showed that the odds of a preterm birth rose, on average, by 5% per 1°C increase in temperature and by 16% during heat waves than on non-heatwave days.

Nearly 2 million stillbirths occur each year worldwide. The average stillbirth rate was 6.2 per 1000 births in the eight included studies, about half the rates documented in many lower-income countries.

All eight studies detected an increase in stillbirths at higher temperatures, with stillbirths increasing by 5% per 1°C rise in temperature. In most cases, associations between temperature and stillbirth were most pronounced in the last week or month of pregnancy.

Low birth weight, associated with a range of short and long-term consequences, occurred on average in 3% of infants in the included studies. Of the 28 studies that assessed birth weight, 18 found an increased risk at higher temperatures, but generally the impacts of temperature on weight were small, with most studies reporting only minor changes.

However, the researchers point out that, "even apparently minor decrements in birth weight could have a major impact on public health as exposure to high temperatures is common and escalating."

Finally, associations between temperature and outcomes were largest among women in lower socioeconomic groups, suggesting that pregnant women in low and middle income countries may be at particular risk from heat exposure, they add.

This review was based on observational studies, of which about a third were of low quality, so no firm conclusions can be drawn about cause and effect. The researchers also point to limitations, such as differences in temperature measures between studies, and the possibility that other factors, such as air pollution, may have had an effect on pregnancy outcomes.

Nevertheless, they say the review included more studies than previous reviews, allowing for comparisons of three outcomes and a more comprehensive assessment of heat sensitivity in pregnancy.

As such, they say the review "highlights the need to identify interventions targeting heat related conditions in pregnant women, especially in women at the age extremes and in lower socioeconomic groups, and to determine their effectiveness."

They argue that pregnant women "merit a place alongside the groups typically considered as at high risk for heat related conditions."

"Given increases in the frequency and intensity of heatwaves, the number of pregnant women exposed to these conditions worldwide, and the significant individual and societal burdens associated with preterm birth and stillbirth, research and policy initiatives to deal with these connections are a high priority," they conclude.