For years, people have had theories on what determines the shifting ratio of baby boys to baby girls, some based on old wives’ tales and others supported by small datasets.
According to a new study of more than 6 million births in the United States and Sweden, changes in the human sex ratio at birth—defined as the percentage of newborns who are boys—are connected with the presence of air and water contaminants, but not with seasonality or weather.
The sex ratio at birth (SRB) can be influenced biologically by hormonal variables that specifically terminate female or male embryos during pregnancy. Previous research has revealed that pollutants, changes in weather, and psychological stress may all affect the SRB, although the majority of studies have concentrated on only one or two aspects at a time.
The researchers studied records from the IBM Health MarketScan insurance claim dataset on more than 3 million births in the United States from 2003 to 2011, as well as records from the Swedish National Patient Registry on more than 3 million births from 1983 to 2013. Other national databases provided additional information on the weather and pollutants at the time of each birth.
Seasons, ambient temperature, violent crime rates, unemployment rates, or commuting time were not linked to variations in the SRB, according to the new research. Several contaminants, on the other hand, were linked to changes in the SRB, with some boosting the ratio of boys and others decreasing it.
Polychlorinated biphenyls (PCBs), iron, lead, mercury, carbon monoxide, and aluminum were found in the air, while chromium and arsenic were found in the water.
Extreme droughts, traffic fatality rates, industrial permits, and empty units in an area were also found to be associated with the SRB.
When the researchers looked at linkages between two stressful events in the United States and the SRB in adjacent places, they discovered no link between Hurricane Katrina and the local SRB, but a strong link between the Virginia Tech shooting and the local SRB.
The study was unable to confirm whether the pollutants were responsible for the observed changes in the SRB.
“Ideally, each SRB-pollutant association could now be followed up with experimental work using human cell lines to dissect the underlying mechanism,” said the lead author of the study Andrey Rzhetsky of the University of Chicago.
The findings may also lead politicians to “decide to make steps toward reducing environmental pollution,” according to the author.
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