The rise in temperatures due to climate change also has effects on birth rates. A study published in PNAS, based on data from five million births in 33 countries in sub-Saharan Africa and India, shows that higher temperatures are associated with a greater risk of miscarriage, particularly for male foetuses.
“This is the first large-scale study to link temperatures recorded during the intrauterine period with individual birth records”, explains Raya Muttarak, a Professor at the Department of Statistical Sciences “Paolo Fortunati” of the University of Bologna and one of the study’s authors. “Thanks to this approach, we were able for the first time to demonstrate, based on large, up-to-date samples from the Global South, that temperature influences the sex ratio at birth.”
It has long been known that shocks experienced during pregnancy can alter the sex ratio at birth, disproportionately increasing miscarriages of male foetuses. In addition, a series of studies have shown that outdoor temperature can affect the frequency of miscarriages. The researchers therefore decided to investigate these aspects through a large-scale analysis that considered data on five million children born between 2000 and 2022 in 33 countries in sub-Saharan Africa and in India.
“The results show that outdoor temperature during pregnancy affects the sex ratio at birth, leading in particular to a decrease in male births”, says Muttarak. “This evidence is consistent with evolutionary theory, according to which males are more vulnerable to adverse prenatal environments and therefore have a higher risk of miscarriage in stressful contexts.”
The researchers also stress that this phenomenon may lead to changes in family planning choices. In India, in particular, the sex ratio at birth has historically been skewed in favour of males due to the practice of female-selective abortion. The increase in miscarriages among male fetuses due to higher temperatures may, however, be reducing this practice and leading to a more balanced sex ratio at birth.
“Rising temperatures have measurable effects on fetal survival and family planning behaviour, with implications for population composition and the balance between sexes” confirms Muttarak. “Understanding these processes is essential for predicting how environmental change will shape the composition of human societies in a warming climate.”
The researchers stress that, in a context of continuously rising temperatures at global level, it is crucial to increase investments to safeguard maternal health and access to healthcare. Climate change is not only an environmental and economic challenge, but also a fundamental issue for public health and demography.
The study was published in PNAS and titled “Temperature and sex ratios at birth”. For the University of Bologna, the study involved Raya Muttarak, a Professor at the Department of Statistical Sciences “Paolo Fortunati”. The other authors are: Jasmin Abdel Ghany, Joshua Wilde and Ridhi Kashyap from the University of Oxford, and Anna Dimitrova from the Barcelona Institute for Global Health (ISGLOBAL).
