December 9, 2021 – Season conception it does not affect whether more boys than girls are born, nor does the ambient temperature, a large study reveals. Similarly, the researchers found no association with the level of violent crime at the site, the unemployment rate, or major events such as Hurricane Katrina.
But certain chemical contaminants were associated with fewer boys 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 more births over 30 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 certainly the largest study to date on environmental factors and their impact on the sex ratio at birth,” says Shanna Swan. dr., which was not related to the research
Variations in the annual birth rate (SRB) – the number of boys born compared to the overall birth rate – are well accepted. It is less clear which things drive these changes.
While not the first study to look for links between major events or pollutants in the air, water and land and SRB, it is the first to explore two very large electronic databases of medical records for responses, senior study author Andrey Rzhetsky, Ph.D. says a professor of medicine and human genetics at the University of Chicago Medscape Medical News.
The findings were published on December 2, 2021 in PLOS computer biology.
And although the SRB did not differ significantly after Hurricane Katrina in 2005, it did after the 2007 shooting at Virginia Tech, Rzhetsky and colleagues found. The SRB was lower than expected 34 weeks after the mass shooting.
Location, location, location
Researchers also found that levels of chemical pollutants “differ significantly” in different regions of the country. For example, lead in the country was elevated in the northeast, southwest, and Middle East of the U.S., but not in the south. Also, the highest levels of total mercury in water samples were found mainly in eastern states, especially in the northeast.
Rzhetsky and coworkers mapped these regional differences in many factors, including hydrazine. Hydrazine is a foaming agent used for the production of pharmaceuticals, agrochemicals and as a propellant for spacecraft.
“Hydrazine appears to follow whimsical stain-like forms in the eastern United States, and each stain is likely focused on a factory that releases this pollutant,” the authors wrote.
To get a more complete picture, the researchers also compared changes in the SRB with data from the US National Oceanic and Atmospheric Administration, the US Environmental Protection Agency, the Swedish Meteorological and Hydrological Institute and Swedish Statistics.
They found that aluminum in the air, chromium in water, and the total mercury level was raised by the SRB. For comparison, lead in the soil and areas with higher occupancy of renters is associated with a lower SRB, ie a higher share of girls born.
Rzhetsky and co-workers also add evidence for a link between polychlorinated biphenyls (PCBs) and SRBs. The previous findings are in conflict, the authors pointed out.
“Because the sample sizes of the studies published so far have been very small, our PCB results would have had significantly greater statistical power,” they said.
Several pollutants had no significant association with SRB in the study, including levels of lead or chromium in the air, arsenic in the soil, and cadmium in the air or water.
The research was said to have had limitations.
“Size is new in terms of birth rates, and statistical methods are unusually sophisticated, but the conclusions don’t really differ from most published ones,” says Swan, a professor of environmental medicine and public health at the institute. Icahn School of Medicine at Mount Sinai, New York.
“The message to take with you is that many of the exposures examined are associated with lower – and some with higher – SRBs is not new, but it is consistent with other, smaller studies,” says Swan, who co-authored September 2021 study assessment of chemicals that disrupt the endocrine system and lower birth rates in Asia.
“Environmental exposure data is quite uneven and is known only at the ecological level, not at the individual level,” she says. “We learn, for example, that SRB has been significantly reduced … among families living in areas with the highest exposure to lead, but also among those among the largest septils of renters’ occupancy rates.”
“It’s hard to assess these mechanisms and credibility,” Swan says.
More research guaranteed
The mechanism is still unknown, but researchers have suggested that the pregnancy of female embryos may end early in development, which will increase SRB. Also, deaths of male embryos are more common in the late second or third trimester, at which point they would reduce SRB. A third factor, maternal hormone levels at conception, could also change SRB.
The links between individual factors and changes in the SRB are precisely that – associations – are not intended to be interpreted as “gender-specific selection mechanisms” that cause differences at this time, the authors noted. Further studies are needed to confirm the association.
The research is a good starting point for future studies to take a closer look at the contributions of pollutants such as arsenic, lead, cadmium and others, Rzhetsky says.