Consistent findings
That said, the research had limits.
“The magnitude is new in terms of number of births, and the statistical methods are unusually sophisticated, but the conclusions don’t really differ from much of what has been published,” says Dr. Swan, a professor of environmental medicine and public health at the Icahn School of Medicine at Mount Sinai, New York.
“The takeaway message that many examined exposures are associated with lower – and some with higher – SRBs is not new but consistent with other, smaller studies,” says Dr. Swan, who co-authored a Sept. 2021 study evaluating endocrine-disrupting chemicals and lower birth rates in Asia.
The data on environmental exposures “is, however, quite uneven, and only known at the ecologic and not the individual level,” she says. “We learn, for example, that SRB was significantly reduced ... among families living in areas with the highest septile of lead exposure but also in those among the highest septile of percent renter occupancy.”
“Evaluating these as to mechanism and plausibility is difficult,” Dr. Swan says.
More research warranted
The mechanism remains unknown, but the investigators suggested that female embryo pregnancies may end early in development, driving the SRB up. Also, male embryo deaths are more common in the late second or third trimester, at which point they would drive the SRB down. A third factor, maternal hormone levels around the time of conception, could also alter the SRB.
The associations between individual factors and SRB changes are just that – associations – not intended to be interpreted as “sex-specific selection mechanisms” causing the differences at this point, the authors noted. Further studies to confirm the associations are needed.
The research is a good stepping off point for future studies to look closer at the contribution of pollutants like arsenic, lead, cadmium, and more, Dr. Rzhetsky says.
A version of this article first appeared on WebMD.com.