Projected Changes in Maternal Heat Exposure During Early Pregnancy and the Associated Congenital Heart Defect Burden in the United States

Authors: Wangjian Zhang Tanya L. Spero Christopher G. Nolte Valerie C. Garcia Ziqiang Lin Paul A. Romitti Gary M. Shaw Scott C. Sheridan Marcia L. Feldkamp Alison Woomert Syni‐An Hwang Sarah C. Fisher Marilyn L. Browne Yuantao Hao Shao Lin and the National Birth Defects Prevention Study Charlotte Hobbs Suzan Carmichael Jennita Reefhuis Sarah Tinker Marlene Anderka Charlotte Druschel Erin Bell Andy Olshan Robert Meyer Mark Canfield Peter Langlois Lorenzo Botto



In the face of unmitigated climate change, the U.S. is seeing an increase in the extreme heat events that are occurring across the country. In fact, the Centers for Disease Control and Prevention (CDC) released a report stating “by the end of this century, extremely high temperatures that currently occur once every 20 years could happen as often as every two to four years” [1]. Extreme heat events can cause severe health impacts such as heat-induced exhaustion, strokes, and even death [2].

Pregnant women are particularly vulnerable to heat stress, as it may cause birth defects such as congenital heart defects [3]. A congenital heart defect (CHD) happens when “the heart, or blood vessels near the heart, don’t develop normally before birth” [4]. Of all the different groupings of types of birth defects, CHDs make up the largest group in terms of how common they are. They are also a “leading cause of infant morbidity and mortality in the U.S.”[5].

Previous studies have shown positive relationships between extreme heat and preterm birth and/or low birth weight. One report in particular associated CHDs with maternal extreme heat exposure during warmer months in certain regions in the U.S. [5]. However, this association had not yet been demonstrated at the national level. This study aimed to bridge that gap and focus on the potential nationwide risk of increasing extreme heat periods and changes in the rates of CHDs that are projected due to climate change.


To expand on the previous regional associations between maternal extreme heat exposure and risk of CHD by projecting the U.S.’s nationwide changes in CHD burden for the years 2025 to 2035 by using multiple heat exposure metric projections.


This study used data about CHD rates in relation to maternal heat stress from a previous study, while also using high-resolution weather projection data to determine expected extreme heat exposure. Seasonal and regional variations were examined, as well as different types of CHD.


Maternal heat exposure
Projections indicate an increase in extreme heat days across the U.S., and thus general increases in maternal heat exposure. The Midwest, followed by the South, and Northeast are projected to have the highest increase in maternal exposure to excessive heat in the summer months of 2025-2035. In the spring highest increases of maternal exposure are projected for the South and Southwest, followed by the Northeast and Midwest.

CHD burden
The Midwest is projected to have the greatest relative increase in the burden of certain types of CHD in 2025-2035, while the South and the Northeast are projected to have the largest number of projected CHD increases overall.


Because of the projected increase in extreme heat days across the U.S., and the association of maternal heat exposure during early stages of pregnancy with CHDs, an increase in the CHD burden is projected for the years 2025 - 2035. In addition, a continuous increase in U.S. births has been projected through 2030, meaning that the projected rates of CHDs could be even higher.


Reduction of greenhouse gas emissions (including carbon dioxide, methane, nitrous oxide, and ozone) is critical in mitigating climate change and the related increase in extreme heat days [6]. The U.S. Environmental Protection Agency (EPA) has been regulating certain greenhouse gas emissions (as air pollutants that threaten human and environmental health) from mobile and stationary sources under the Clean Air Act (CAA). Examples include: 1) the 2012 Clean Car Standards which established greenhouse gas reduction and fuel efficiency standards for new cars and passenger vehicles in model years 2017-2025 [7]; 2) the 2015 Clean Power Plan which set limits on carbon emissions that come from coal-fired power plants, and considered by some to be the “single-biggest step we can take to fight climate chaos” [8]; and 3) the 2016 New Source Performance Standards (NSPS) and Permitting Rules for the Oil and Natural Gas Industry as part of the strategy to reduce methane emissions [9].

These rules and standards are currently being pressed for revision and roll-backs by the current administration, and efforts to weaken the CAA are underway. It is imperative that the government uphold EPA’s 2009 Endangerment Finding, which concluded that greenhouse gas emissions do harm human health and need to be regulated under the CAA. The rules and regulations promulgated under this act need to be protected and strengthened to mitigate the health threats of climate change.

In addition, improved measures to reduce extreme heat exposures to vulnerable populations are needed. Currently, the Occupational Safety and Health Administration (OSHA) has no standards for heat exposure in the workplace [10]. Rather, the agency provides non-enforceable guidelines to help employers and supervisors prepare and implement hot weather plans. Because extreme heat is projected to become an even larger problem than it is now, OSHA at the federal and state levels need the resources and capacity to either ensure these guidelines are being followed or to set standards that take pregnant employees into consideration, and ensure compliance via enforcement.

It is also essential that state and local governments have the funding, training, and capacity to effectively plan for and protect its residents during periods of excessive heat in order to best protect human health, especially that of maternal and child health.

Finally, the health impacts of climate change disproportionately burden lower-income communities and communities of color. These under-resourced groups are often the first to feel the impacts of climate change and the last to receive mitigation efforts, and mitigation efforts can exacerbate existing disparity. All mitigation strategies need to be inclusive from the start, to be informed and shaped by those most impacted, to ensure equitable benefit.


1. CDC. (2013). Climate Change and Extreme Heat. Retrieved March 4, 2019 from

2. National Institute of Environmental Health Sciences. (2017, July 20). Effects of Heat. Retrieved March 4, 2019 from

3. CDC. (2017, April 20). Reproductive Health and the Workplace. Retrieved March 4, 2019 from

4. American Heart Association. (2018, May 8). About Congenital Heart Defects. Retrieved March 4, 2019 from

5. Zhang, W., et. al.. (2019, Jan. 30). Projected Changes in Maternal Heat Exposure During Early Pregnancy and the Associated Congenital Heart Defect Burden in the United States. Retrieved March 4, 2019 from

6. NASA. (2019, Feb. 27). A Blanket Around the Earth. Retrieved March 4, 2019 from

7. Environmental Defense Fund. (2019). Federal clean car standards. Retrieved March 20, 2019 from


9. United States Environmental Protection Agency. (2017, April 27). New Source Performance Standards and Permitting Requirements. Retrieved March 20, 2019, from

10. United States Department of Labor. (2019). Overview: Working in Outdoor and Indoor Heat Environments. Retrieved March 4, 2019 from

Article found in Journal of the American Heart Association.