Sperm counts have declined worldwide over the last few decades, dropping over 60% since 1972, according to a new meta-analysis published in Human Reproduction Update—findings that could potentially impact global fertility going forward.

Study details and key findings

For the meta-analysis, researchers analyzed 223 studies from around the world, updating a prior analysis their team published in 2017. The new study adds data on sperm counts for 2011 to 2018, as well as data from new regions, including South America, Asia, and Africa.

Overall, the researchers found that sperm counts had "declined appreciably" between 1973 and 2018 on all of the studied continents, with total sperm count falling 62.3%. Since 1972, total sperm count has declined by 1.4% per year. However, the rate of decline since 2000 has been much faster, decreasing 2.64% each year.

Average sperm concentration also fell during the same period, going from 101.2m per ml to 49m per ml—a 51.6% drop. According to previous research, fertility may be compromised if sperm concentration is below 40m per ml.

Although the estimate in the current study is above this threshold, Hagai Levine, the study's lead author from the Hebrew University of Jerusalem, said the percentage of men below this threshold has likely increased over time.

"Such a decline clearly represents a decline in the capacity of the population to reproduce," he said.

Currently, it is not known why sperm counts are falling, but Shanna Swan, one of the study's authors from the Icahn School of Medicine, said that a major factor could be endocrine-disrupting chemicals. These chemicals, which include phthalates and bisphenols, are found in thousands of everyday items, including water bottles and takeout containers.

Other potential risk factors for lower sperm counts include poor diet, smoking, obesity, stress, and binge drinking.

Going forward, "[r]esearch on the causes of this continuing decline and actions to prevent further disruption of male reproductive health are urgently needed," the study authors wrote.

Commentary

According to Levine, the study's findings "serve as a canary in a coal mine" and "if not mitigated, could threaten humankind's survival."

"I think this is another signal that something is wrong with the globe and that we need to do something about it," Levine said. "So yes, I think it's a crisis that we [had] better tackle now, before it may reach a tipping point which may not be reversible."

Because of the accelerated decrease in global sperm counts, Swan said that "more people will need to use assisted reproduction to conceive." In addition, she noted that lower sperm counts have been "linked to more disease later in life—cardiovascular disease, diabetes and reproductive cancers—and a shorter life expectancy."

However, other researchers have remained more cautious in their assessment of the study's findings, noting that while sperm counts have declined, it may not be enough to actually affect the human population. There are also several other factors besides sperm count that affect fertility.

The study is not "suggesting that our sperm concentrations are crashing at a rate that is going to lead us out to every man needing to walk into an infertility center," said Amy Sparks, a reproductive physiologist at the University of Iowa who was not involved in the study.

"We can't ignore that things are changing. Lifestyles have changed. Dietary patterns certainly have changed," Sparks said. "Through these changes, we are exposing our body to altered conditions. In response to those altered conditions, we're seeing a decrease in sperm concentration."

So far, "[w]e don't understand why we're seeing this pattern, so I think it's hard to be alarmist for an individual," said Michael Eisenberg, a urologist who focuses on male fertility and sexual function at Stanford University and Stanford Health Care. "But at a policy level, this should be a wake-up call to try and understand." (Weintraub, USA Today, 11/15; Elbein, The Hill, 11/15; Wei, Bloomberg, 11/16; Davis, The Guardian, 11/15; Neville, Financial Times, 11/15; Levine et al., Human Reproduction Update, 11/15)