THURSDAY, Nov. 5, 2020 — Global warming could increase rates of serious pregnancy problems, researchers warn.
They analyzed 70 studies from 27 countries that reported associations between high temperatures and preterm birth, birth weight and stillbirths.
“Given increases in the frequency and intensity of heat waves, 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,” wrote study co-author Matthew Chersich and colleagues. Chersich is a research professor at the University of the Witwatersrand in Johannesburg, South Africa.
Preterm birth — when a baby is born before the 37th week of pregnancy — is the leading cause of death among children younger than 5 years. Each year, 15 million babies worldwide are born preterm, according to the World Health Organization.
Of the 47 studies that assessed preterm births, the average rate was 5.6%, much lower than the global average of about 10%. Forty of the studies found that preterm births were more common at higher than lower temperatures. The risk of preterm birth rose, on average, by 5% per 1° Celsius (1.8° Fahrenheit) increase in temperature and by 16% during heat waves versus on non-heat wave days.
Nearly 2 million stillbirths occur worldwide each year. Among the eight studies that examined stillbirth, the average rate was 6.2 per 1,000 births, with about half of the rates in many lower-income countries.
All eight studies found an increase in stillbirths at higher temperatures, with stillbirths increasing by 5% per 1° Celsius 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 is associated with a range of short- and long-term consequences. Of the 28 studies that assessed birth weight, the average rate of low birth weight was 3%. Eighteen of the studies found an increased risk of low birth weight at higher temperatures, but most reported only minor effects.
However, “even apparently minor decrements in birth weight could have a major impact on public health as exposure to high temperatures is common and escalating,” according to the authors of the review, which was published online Nov. 5 in the BMJ.
The researchers also found that associations between temperature and pregnancy outcomes were largest among lower-income women. This suggests that pregnant women in low- and middle-income countries may be at particular risk from heat exposure, the study authors said in a journal news release.
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