There are growing fears that modern life is negatively impacting human fertility, and it is a common theme of discussion online.
The viral article titled "Sperm Counts Drop by 62% Worldwide" claims that "sperm counts worldwide plunged by sixty-two percent in under fifty years between 1973 and 2018, and this could lead to a reproductive crisis."
The article references a study published in the journal Human Reproduction Update on November 15. This study is a meta-analysis of results from 223 studies, yielding 288 estimates based on semen samples collected 1973–2018.
Further posts on Reddit and other platforms, as well as reports in the media, cited the same study, gathering hundreds of thousands of social media engagements. Some have linked the results to the COVID pandemic, without offering evidence in support.
The Human Reproduction Update paper concluded that worldwide sperm counts (measured both as sperm concentration per ml and total sperm count) decreased significantly between 1973 and 2018.
The paper states: "Using TSC [total sperm count] model estimates of 335.7 million in 1973 and 126.6 million in 2018, TSC declined among unselected men by 1.40% per year and 62.3% overall." In this context, "unselected" refers to men involved in studies where they were not selected by fertility status, such as young men unlikely to be aware of their fertility being screened for some other reason than trying to conceive.
"The researchers published some of this aggregated data previously (2017), but included 38 newer studies (14,233 samples) from 2014–2019, importantly from all areas of the globe," Sarah Martins da Silva, a senior lecturer in reproductive medicine and male infertility research at the University of Dundee in the U.K., told Newsweek.
"Their findings conclude that sperm counts are falling by around 1.1 percent per year, with an overall decline of 51.6 percent in 45 years. Of concern, the rate of decline appears to have doubled since 2000."
Martins da Silva also stresses that while critics of this study might cite that we count sperm differently to 40 years ago, all studies in this publication used the same way of counting sperm (haemocytometer).
"And the authors only included published data for men with known fertility or unselected for fertility," she said. "Men that were infertile, taking medication or with medical conditions that may affect their fertility, smokers, etc were all excluded. Not everyone believes the data, but my conclusion is that sperm counts are falling."
Alex Ford, an ecotoxicologist at the University of Portsmouth in the U.K., agrees.
"In my opinion, there is now unequivocal evidence that sperm counts are declining and increasing evidence that this could be due to chemical exposures during early development," he told Newsweek.
"These claims started from research about 20 years ago but there were suggestions that the research at the time had various limitations and did not take into account lifestyle, diet, exercise and the methods for determining sperm counts are variable by country and changed over time.
"What these researchers have been able to do is disentangle all the confounding factors by looking at hundreds of studies over decades and they still come to the same declining sperm counts across many world continents.
"A boy born today will have half the sperm of their grandfather as sperm counts are dropping by 1-2 percent per year."
There is a wide range of chemicals that are known to interfere with the endocrine system and act like hormones, many of which are found in plastics, cleaning products, and pesticides, Ford warned.
"It is thought that during critical stages in the development of an organism/human, if the endocrine signaling is interfered with then the effects can manifest themselves later in life," Ford said.
Martins da Silva agreed: "Exposure to pollution, plastics, smoking, drugs and prescribed medication, as well as lifestyle, such as obesity and poor diet, have all been suggested to be contributory factors although effects are poorly understood and ill-defined."
While these comments address some of the concerns about the meta-analysis, others still remain, fueling skepticism among fellow researchers.
First, there is some debate about the correlation between sperm counts and fertility implied in the analysis, which other research, including an April 2022 paper published in response in the British Columbia Medical Journal (BCMJ), has cast doubts over.
As long as people have a concentration of at least 40 million sperm per milliliter of semen, having additional sperm doesn't increase fertility, according to the World Health Organization's 2010 report.
This latest paper found average sperm concentrations have dropped from 104 to 49 million per ml on average, which would still put it over that critical threshold.
"We don't see [sperm count] predicting much of anything," Germaine M. Buck Louis, a reproductive epidemiologist at George Mason University, told the New York Times back in June 2021.
As mentioned above, the researchers' previous meta-analysis was criticized by various other scientists in the field, with the BCMJ paper cited earlier stating "the data remain highly heterogeneous with immeasurable confounding biases that cannot be addressed with currently available retrospective data."
Harvard's GenderSci Lab, meanwhile, argued that the assumptions underlying this analysis are scientifically and ethically problematic. The authors claimed that the original 2017 study "relied on racist and colonial hierarchies and assumptions because it categorized data as "Western" sperm counts or "Other" sperm counts."
That criticism has arguably been mitigated by the widening geographic scope of the follow-up analysis, but without further analysis and feedback from the scientific community it is hard to gauge what impact these improvements had on the accuracy of the work.
Furthermore, meta-analysis as a methodology has come under scrutiny in various fields. As Newsweek has previously mentioned, meta-analysis is a type of statistical analysis that combines and compares the results of multiple scientific studies.
While this type of study in medicine may provide a consolidated and quantitative review of a large and often complex body of literature, it also has its drawbacks.
For example, "a failure to identify the majority of existing studies can lead to erroneous conclusions," author A. B. Haidich warned in a December 2010 paper assessing the pros and cons of this approach.
Although that in itself does not negate the outcome of this particular study, it is another caveat that needs to be taken into consideration.
Finally, the ensuing social media posts linking the results of this study with COVID-19 are provably false, as the data range of the sampled papers—1973 to 2019—predates the pandemic.
The paper published in the journal Human Reproduction Update does conclude that across 14,000 sperm samples involved in a number of independent studies, total sperm count was found to have declined by 1.4 percent per year and 62.3 percent overall between 1973 and 2018.
Additionally, the rate of decrease appears to be accelerating, according to the research.
However, issues with the meta-analysis methods and debate about the true implications of sperm count on fertility mean that these results don't amount to definitive proof that human fertility is or will become in decline, as some who shared the piece appeared to interpret.
FACT CHECK BY NEWSWEEK
Levine, H., et al. Temporal trends in sperm count: a systematic review and meta-regression analysis of samples collected globally in the 20th and 21st centuries, Human Reproduction Update, 2022. https://doi.org/10.1093/humupd/dmac035
Tong, N., Global decline of male fertility: Fact or fiction? BCMJ, 2022.