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Kieran Lewis, Rossella Cannarella, Fangzhou Liu | Fertility and Sterility | (2024)
Primary Link Archive Pubmed DOI Openalex Full text (OA)

Key Takeaways

Plain English Takeaway

Sperm counts in American men who are able to have children have not dropped over the past 50 years, so worries about a big decline may be overstated.

Study Aim

The main goal of this paper is to find out if sperm concentration (the number of sperm in a given amount of semen) has changed over time among American men who are fertile or have no known fertility problems. The authors want to address concerns raised by earlier studies that suggested sperm counts are falling worldwide. Simply put: The study wants to see if sperm counts in American men who can have children have gone down over the years.

Study Design

The researchers performed a systematic review and meta-analysis (a study that combines results from many other studies) of published research from 1970 to 2023. They searched scientific databases for studies that measured sperm concentration in American men without known infertility. In total, they included 58 studies, covering 75 different groups and 11,787 men. They used statistical models to look for trends over time and also checked if results differed by region or fertility status. Simply put: The authors looked at lots of studies from the last 50 years to see if sperm counts in American men have changed.

Findings

The study reveals that, overall, there was no significant change in sperm concentration among American men from 1970 to 2018. Even after adjusting for region and fertility status, any decline found was very small and not likely to matter in real life. In fact, when looking at total sperm count (the total number of sperm in a sample), there was a small but significant increase over time. The authors conclude that, unlike some global studies, there is no strong evidence of a major drop in sperm counts among fertile American men or the general male population without infertility. They recommend more research to keep monitoring this issue. Simply put: The research shows sperm counts in American men who can have children have stayed about the same for 50 years.

Abstract

IMPORTANCE: Findings from several high profile meta-analyses have raised concerns about an ongoing global decline in sperm concentration and male fertility. However, these studies exhibit considerable heterogeneity in key variables including study population, methodology, fertility status, and geographic region. OBJECTIVE: To perform a systematic review and meta-analysis exploring temporal trends in sperm concentration among fertile men and men unselected for fertility status in the United States. DATA SOURCES: A literature search performed in Scopus and PubMed databases for studies published between 1970 and 2023. Additional studies were included from citations of prior global meta-analyses and reviews evaluating temporal trends in sperm count. STUDY SELECTION AND SYNTHESIS: Studies were included if they presented original data on sperm concentration in US men without known infertility from 1970 to 2023. Aggregate data were assessed across all study populations, with additional subgroup analyses stratified by fertility status and US region. MAIN OUTCOMES: Weighted generalized linear models were generated to evaluate the association between mean sperm concentration and sample collection year. RESULTS: A total of 874 articles were screened, with 58 meeting the inclusion criteria. These represented 75 unique study populations totaling 11,787 men in the United States. Across all study populations, no change in sperm concentration was observed between 1970 and 2018 in unadjusted models (β = 0.14 million/mL per year). When adjusting for US region, no statistically significant decline in sperm concentration was seen. When adjusting for both region and fertility status, a modest annual decline was observed to meet statistical significance (β = -0.35 million/mL per year). Of the 49 study populations reporting adequate data to determine mean total sperm count, there was a significant increase in total sperm count of 2.9 million per year between 1970 and 2018. Subgroup analysis found no statistically significant change in mean sperm concentration among any US census region or fertility status cohort. CONCLUSION AND RELEVANCE: In contrast to prior global studies, this analysis suggests no clinically significant decline in sperm concentration among confirmed fertile men and the general male US population without known infertility. Although these findings provide some reassurance against a widespread rapid decline, further studies are necessary to better understand this important topic.

Referenced In

Lee Johnson
a month ago
A Scientific Review of Claims made by RFK Jr.

Claim: Men in 1970 Had Twice the Sperm Count of Teenagers Today

Verdict: Possibly true, but misleading

“We have, as Dr. Oz pointed out, a fertility crisis in this country right now. We just found out that we've dropped out of 1.57 percent. […] The fertility crisis for women began in 2007. For men: in 1970, men had twice the sperm count, as our teenagers do today.”

www.youtube.com

RFK Jr. spoke about the fertility crisis last week at a maternal health event in the White House, and in the process made this bizarre-sounding claim about male sperm counts. Is it true? Is this what we should focus on in the fertility crisis?

What RFK Jr’s Source Actually Says

RFK Jr’s comment is specifically in reference to this meta-analysis, which compares sperm counts for men in various global regions between 1973 and 2018.

His comment is broadly in line with the results of the study. The researchers found that men from North America, Europe and Australia had seen roughly 50% declines in sperm count from 1973 to 2018.  

Note two things:

  1. The study didn’t only look at Americans – US data was combined with other regions.

  2. The study didn’t look at teens specifically; the data wasn’t broken down by age.

Based on the analysis, the lowest possible sperm count for North American men would be around 46 million per millilitre. However, the paper also notes that beyond a threshold value of somewhere around 40-50 million/ml, a higher sperm count doesn’t necessarily mean you’re more likely to conceive.

Other Papers Don’t Agree

Another meta-analysis directly challenged the results above, focusing on just the U.S. and finding that there is no significant decline in sperm concentration for men without known infertility. The authors note that results like the one above may have been biased by infertile people, subfertile people and people at risk of infertility, while in other men, there is no difference.  

The U.S. Fertility Crisis and Sperm Count

There are many factors causing the decline in fertility across much of the world, mainly cultural in nature (for example, expanding reproductive rights) or simply a result of less childhood mortality. The problem exists throughout Europe, parts of Asia and Australia too, and in fact even Canada has lower fertility than the U.S..

Sperm counts have declined, but they are not the primary reason for the fertility crisis.

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