July 23, 2019
It's shaping up to be a sweltering summer. A dangerous heat wave blanketed large swaths of the country last week, and we're not alone: last month was the hottest June ever recorded across the globe.
Medical professionals have taken note and are voicing their growing concern. Just last month, over 70 medical and public health groups declared climate change a public health emergency and one of the greatest health threats the country has ever faced.
There is ample evidence to support their claims: rising temperatures exacerbate respiratory, cardiovascular, and infectious diseases, and trigger eco-anxiety. But climate change does not impact everyone equally: the most vulnerable groups — children, the elderly, people with disabilities, and pregnant women — are disproportionately at risk.
Yet for some groups, the full implications of rising temperatures are only beginning to be investigated. "There have been limited studies on the populations we think are most vulnerable, including pregnant women," said Greg Wellenius, ScD, director of the Center for Environmental Health and Technology at Brown University in Providence, Rhode Island.
Patrick Kinney, ScD, professor in the Department of Environmental Health at Boston University's School of Public Health in Massachusetts, agrees. "Temperature as a risk factor [for pregnancy outcomes] is something people have begun looking at only recently," he told Medscape Medical News. "It's a small literature so far, but growing rapidly as people become more aware of the impact of climate change."
A number of studies have looked at the impact of temperature on birth weight, with mixed results. A 2016 study examined birth certificate data in New York City between 1985 and 2010, and found that exposure to one extra day of extreme temperatures, whether hot or cold, was associated with a reduction in birth weight. A 2017 study using data from 12 US sites showed a similar result.
But other studies found associations between birth weight and only colder or warmer temperatures, and some found no association at all between ambient temperature and birth weight.
Now, an observational study published last month in Environmental Health Perspectives uses the largest dataset to date to try and tease out the association. "We're trying to more comprehensively and thoroughly understand the health impact of continued climate change," said Wellenius, senior author of the study. "Prior studies have given different results, and one of the goals here was to use a very large dataset to more conclusively answer this question."
The researchers used live birth data from the CDC's National Center for Health Statistics between 1989 and 2002, and restricted their analyses to singleton births between 37 and 44 weeks of gestation. In all, nearly 29.6 million records were analyzed, and infants with birth weights below the tenth percentile for their gestational age were classified as small for gestational age (SGA).
As there was no way of collecting exact temperature data for each pregnancy, the researchers inferred the daily ambient temperature from spatially interpolated weather data, reflecting "a more spatially explicit representation of meteorological exposures than observations at individual weather stations," they write.
For each day and in each county, they calculated population-weighted temperature averages. Finally, for each birth, temperatures were averaged during each trimester and for the entire pregnancy.
"The main finding is that as temperatures increase for any particular location, you see a slight increase in [the] risk of a baby being born small for gestational age," said Wellenius.
Specifically, temperatures above the 90th percentile for a given county-and-trimester distribution were associated with 1.041 (95% confidence interval, 1.029 - 1.054) higher odds of SGA relative to temperatures in the 40th to 50th percentile. Temperatures colder than the average did not affect risk of SGA.
It's important to note that the researchers measured variation from the mean for a given location at a given time of year, rather than absolute temperature. "The warm weather — when it's unusually hot — might have a bigger impact on people that don't have air conditioning and haven't adapted physiologically or behaviorally to those temperatures," Wellenius explained.
As to whether these findings might apply outside the United States, he advised prudence. "One has to always be cautious when extrapolating, particularly to a different country where something like housing conditions, the availability of air conditioning, [and] the availability of healthcare varies significantly," he said.
And while the result is statistically significant, the clinical significance is harder to qualify.
"It's important to contextualize the odds ratio," said Sarah Rae Easter, MD, fellow in maternal-fetal medicine at Brigham and Women's Hospital in Boston, Massachusetts. "From the standpoint of a clinician, I think focusing attention on known and modifiable risk factors for fetal growth restriction may be a better investment in improving pregnancy outcomes."
For his part, Kinney said the findings could be clinically important. "Even though birth weight itself isn't necessarily a serious concern, it's associated with a lot of adverse outcomes later in life," which increases the significance of the findings.
Birth weight isn't the only outcome being examined though. A 2014 study found a positive association between increased temperatures and a lower male-to-female sex ratio of newborn infants in Japan.
In all these studies, however, ambient temperature is an imperfect measure of the subjects' lived environments. "We don't have data on things we would like to know, like how much time was spent outside," said Pauline Mendola, PhD, an investigator in the Epidemiology Branch at the Eunice Kennedy Shriver National Institute of Child Health and Human Development. "We don't know what kind of heating or cooling people have, how much actual exposure to the ambient temperature people have."
Mendola and her colleagues published an analysis of stillbirths as a function of temperature in 2017 that found that whole-pregnancy exposures to extreme temperatures were associated with higher stillbirth risks.
The possible mechanisms behind the adverse effects of extreme temperatures remain poorly understood. The authors cite several studies linking warmer-than-average and colder-than-average temperatures with markers of oxidative stress and systemic inflammation, alterations in placental oxidative capacity, and changes in blood viscosity and uterine blood flow.
Regardless of the underlying mechanisms, it doesn't hurt to advise caution when possible, Mendola said. "We may not need to understand placental function or vascular response to be able to give some recommendations.... The reasonable thing to suggest [to a patient] is if you have extreme temperatures for your area, then avoid them."
Even if the full effect of temperature on pregnancy outcomes is never understood, it's likely that any solution would have to be part of a greater strategy to tackle the root cause. "Climate change is something that we're all thinking about, and perhaps the scenario of pregnancy makes it a little more evident," Easter said. "Most pregnant women and their families are thinking about not only getting through their pregnancies, but also the world they are going to bring their children into."
The authors and interviewees have disclosed no relevant financial relationships.