Humans, animals, plants and other living beings need a very specific climate in order to live. The intensifying climate change we have been witnessing in recent decades has destroyed animals and plants and caused a loss of biodiversity that have been linked to the spread of infectious diseases. 

Air pollution from burning fossil fuels can lead to respiratory and heart conditions, which are linked to an increased risk of death from COVID-19. Extreme weather events spurred by the climate crisis, including hurricanes, wildfires, and record-breaking heatwaves, can result in illness and death, especially among vulnerable populations.

Now, a nationwide study in Israel of more than 600,000 births reveals consistent links between high and low temperatures and newborns’ low birth weight especially when they were exposed to heat during the second and third trimester of pregnancy. 

Just published in the journal Environmental Health Perspectives by a researcher at Bar-Ilan University’s Azrieli Faculty of Medicine in Safed, the study evaluates critical windows of susceptibility to extreme low and high temperatures during pregnancy and fetal growth in a nationwide cohort of 624,940 singleton term births in Israel during the period 2010-2014.  

The study, entitled “Low and High Ambient Temperatures during Pregnancy and Birth Weight among 624,940 Singleton Term Births in Israel (2010–2014): An Investigation of Potential Windows of Susceptibility,” was led by Dr. Keren Agay-Shay, director of the Health & Environment Research Lab at the faculty, together with researchers from Bar-Ilan’s department of geography and environment, the University of Haifa, Ben-Gurion University (BGU) of the Negev in Beersheba, the Israel Meteorological Service and ISGlobal, Universitat Pompeu Fabra amd CIBER Epidemiologia y Salud Publica, all three located in Spain.

Because of rising temperatures resulting from climate change, the researchers stressed that public health organizations should develop strategies to cope with these challenges on a national and a community, level. In addition, identifying windows of vulnerability to temperature could help gynecologists in preparing and refining a set of recommendations they provide to pregnant women.

The researchers collected, mapped, modeled and analyzed data on all recorded live births, residential information at the time of birth, and outdoor temperatures throughout the pregnancy. They subsequently divided the country into three climatic zones and performed a statistical analysis comparing all the data during the days, weeks and trimesters throughout the pregnancy.

Birth certificate data for all live births was obtained from the national birth and birth defect registry in Israel’s Health Ministry. These data include infant birth outcomes, parental sociodemographic characteristics and residential address at the time of birth. 

Birth addresses converted into geographic coordinates and the Israeli property-mapping database. Daily temperatures were estimated, based on data from the Israel Meteorological Service. Maternal residence address at birth, as well as estimated date of last menstrual period, were used to assign estimates of daily mean, minimum and maximum temperatures during each day of pregnancy.

Israel’s highly variable climate, determined by altitude, latitude and proximity to the Mediterranean Sea, was classified into three climatic zones – Mediterranean (characterized by a dry, hot summer), and semi-arid and desert/arid, both characterized by dry climate and hot temperatures. 

For each zone, average daily temperatures during various time periods –such as days, weeks, trimesters and the entire pregnancy – were converted into percentiles based on the distribution among all women in the cohort who resided in the climatic zone. 

Next, the researchers performed a statistical analysis and evaluated the associations between temperature centiles and birth weight adjusting for newborn ethnic-religious group, gender and maternal marital status, origin, age, education, socioeconomic class and employment status. 

Consistent associations were observed between high and low temperatures and fetal birth weight. The strongest links were observed during the fourth through sixth month and particularly the last three months of pregnancy for heat and cold. The researchers found that the coldest and warmest temperatures were associated with lower term mean birth weights than were temperatures near the middle of each distribution. 

“Our study demonstrated the significant associations between exposure to high and low outdoor temperature and birthweight in all term births born in Israel during five years,” related Afay-Shay. Lower birthweight may indicate abnormalities in intrauterine growth and is a risk factor for medical problems during early childhood and over the entire life course.

The identification of susceptible “windows” during pregnancy may clarify underlying mechanisms and potential strategies to reduce risks in pregnant women exposed to extreme temperatures. Study estimates suggest a potential window of vulnerability to cold from three to 36 weeks, with the strongest association during week 36, and potential windows of vulnerability to heat from weeks three to nine and 19 to 34.