Extreme heat events and fertility: a national study

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PROJECT SUMMARY/ABSTRACT The frequency and intensity of heatwaves and other extreme weather events is increasing rapidly owing to climate change and is only projected to escalate in the coming decades. While the hazards of extreme heat on human health are widely recognized, including adverse effects on pregnancy and birth outcomes, little is known regarding ambient temperature and human fertility. Demographic studies suggest that hot weather causes a significant decline in birth rates 8 to 10 months later, yet the drivers of this association are unclear. The animal literature has also long documented a link between maternal hyperthermia induced by high ambient temperatures and reduced fertility, largely mediated through effects on oocyte developmental capacity; however, whether heat stress has a similar impact on follicular development in women is less clear. The proposed research seeks to use a large existing database and robust methodological approaches to test the overarching hypothesis that women exposed to extreme heat during folliculogenesis will have compromised oocyte quality and embryo development, higher risk of implantation failure, and lower probability of live birth. We also hypothesize that stronger effects will be observed following heat events of longer duration and greater intensity and that effects will vary by temporal-, spatial-, and individual-level attributes. To test this hypothesis, we will utilize data from over 2 million non-donor assisted reproductive technology (ART) cycles initiated in the US from 1996 to 2016 that are collected as part of the National ART Surveillance System. National weather data will be integrated using publicly available data from the High Resolution Land Data Assimilation System– a national 1 km resolution dataset of near-surface temperature and humidity that spans 1981-2016. Women undergoing ART represent an ideal setting to investigate the hypothesis that extreme heat affects fertility through adverse effects on the ovary as we can determine exact periods of extreme heat exposure and directly observe early reproductive outcomes that would never be observed in couples conceiving without assistance. By restricting our analysis to cycles using donor sperm and frozen embryo transfers, we can also uniquely examine the effects of extreme heat exposure during folliculogenesis on ART outcomes independent from heat exposures to the male partner and during the implantation window. Given the parallel trends of rising temperatures and the increasing number of women delaying motherhood until 35 years and older, understanding the effects of extreme heat on fertility is becoming increasingly important. Our findings can greatly inform targeted regulations, policies, public health warning systems, and interventions with the ultimate goal of reducing infertility and early pregnancy loss.