The season of conception does not affect whether more boys than girls are born, nor do temperatures in the environment, a large study reveals. Similarly, researchers found no connection with a location’s violent crime level, unemployment rate, or major events like Hurricane Katrina.
But certain chemical pollutants were related to fewer boys being born compared to girls when researchers looked at data for more than 3 million newborns over 8 years in the U.S. and another 3 million born over 30 years in Sweden.
“With data on births in 150 million people in the U.S. over 8 years and 9 million Swedes over 9 years, this is almost surely the largest study to date on the question of environmental factors and their influence on sex ratio at birth,” says Shanna Swan, PhD, who was not affiliated with the research
Variations in the annual sex birth ratio (SRB) – the number of boys born compared to the total birth rate – are well-accepted. Less clear is what things drive these changes.
Although not the first study to look for connections between major events or pollutants in the air, water, and land and the SRB, it is the first to mine two very large electronic medical record databases for answers, senior study author Andrey Rzhetsky, PhD, a professor of medicine and human genetics at the University of Chicago, tells this news organization.
The findings were published Dec. 2, 2021, in PLOS Computational Biology.
And even though the SRB did not vary significantly after Hurricane Katrina in 2005, it did after the 2007 shooting at Virginia Tech, Dr. Rzhetsky and colleagues found. The SRB was lower than expected 34 weeks after the mass shooting.
Location, location, location
The researchers also found the levels of chemical pollutants “varied remarkably” across different regions of the country. For example, lead in the land was elevated in the Northeast, Southwest, and Mideastern U.S. but not in the South. Also, the highest levels of total mercury in water samples was found mostly in Eastern states, especially in the Northeast.
Dr. Rzhetsky and colleagues mapped these regional differences in many factors, including hydrazine. Hydrazine is a foaming agent used to make pharmaceuticals, agrochemicals, and as a propellant for spacecraft.
“Hydrazine appears to follow capricious, blotch-like shapes in the eastern U.S., each blotch likely centered at a factory emitting this pollutant,” the authors wrote.
To get a more complete picture, the investigators also compared changes in the SRB to data from the U.S. National Oceanic and Atmospheric Administration, U.S. Environmental Protection Agency, Swedish Meteorological and Hydrological Institute, and Statistics Sweden.
They found that aluminium in air, chromium in water, and total mercury levels drove the SRB up. By comparison, lead in soil and areas with a higher renter occupancy were linked to a lower SRB, or a higher proportion of girls being born.
Dr. Rzhetsky and colleagues also add to the evidence for a link between polychlorinated biphenyls (PCBs) and the SRB. Previous findings conflict, the authors noted.
“Since the sample sizes of the studies published thus far were very small, our PCBs result would have substantially larger statistical power,” they said.
Several pollutants had no significant link to SRB in the study, including levels of lead or chromium in the air, arsenic in the soil, and cadmium in the air or water.