Number of Baby Boys Declining, Study Uncovers Unexpected Reason
A recent study has found that rising environmental temperatures influence a decrease in the number of male babies compared to female babies. Research by a team from Oxford University shows that heat above 20 degrees Celsius can affect the birth sex ratio between male and female babies.
For years, the sex ratio at birth, representing the proportion of male and female babies, has been considered relatively stable and determined genetically. However, new research published in the Proceedings of the National Academy of Sciences indicates that environmental factors, including extreme temperatures, can also influence birth composition.
In the study, researchers analysed data from more than five million births across 33 sub-Saharan African nations and India. The results showed that exposure to high temperatures can increase the risk of foetal death in early pregnancy, particularly in male foetuses.
One researcher, Abdel Ghany, stated that environmental temperature can affect human reproductive processes fundamentally. “Temperature can determine who ultimately is born and who does not survive during pregnancy,” he said. He noted that the findings demonstrate that temperature has measurable consequences for foetal survival and family planning behaviour, with implications for population composition and gender balance.
“Understanding these processes is essential to anticipating how the environment affects society in a warming climate,” he added.
The research identified a temperature threshold of approximately 20 degrees Celsius as the point where changes in birth ratios begin to appear. When temperatures exceed this figure, the number of male babies born tends to be lower.
Previous research has also found that heat exposure during pregnancy can impair a mother’s ability to regulate body temperature. This condition increases the risk of miscarriage, particularly if the mother experiences dehydration, reducing blood flow, oxygen, and nutrient supply to the foetus.
Beyond biological impacts, high temperatures can also affect family decisions regarding pregnancy. Researchers note that extreme temperatures can trigger economic uncertainty or hinder mobility, thereby affecting access to reproductive health services.
The impact of heat is not evenly distributed. Women with limited resources or living in vulnerable areas are said to be at greater risk of health impacts from high temperatures.
These findings are receiving attention amid rising global temperatures due to climate change. The World Meteorological Organization noted that 2024 was one of the periods with the highest number of heat stress days and tropical nights on record.
Researchers believe these findings demonstrate that climate change impacts not only the environment but also human reproductive health and future population composition. Therefore, further research is needed to understand environmental factors affecting pregnancy and to improve maternal health protection in an increasingly warming world.
In 2024, many European nations reported their lowest birth rates in several decades. Whilst experts state that 2.1 children per woman is needed to maintain stable population size, some countries have consistently shown rates below 1.5.
A recent systematic review by researchers at Catholic University of Chile found that increasing climate-related events and natural disasters significantly disrupt reproductive processes from conception to childcare, including reproductive intentions, pregnancy, childbirth, fertility, and child-rearing.