Not all is well when it comes to men's sperm counts across the Western world.

A new, wide-reaching study published in the journal Human Reproduction Update, has found that sperm counts among men in the U.S., Europe, Australia, and New Zealand have declined by about 50 percent in the last 40 years.

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For the new research, scientists reviewed 185 studies published between 1973 and 2011. In total, these studies were based on 42,000 people who provided sperm samples.

The new study found that there has been a 50 to 60 percent decline during this period in the total sperm count among men who participated in the studies the authors examined.

The paper's authors say the decline in sperm count shows no signs of easing, and the results imply that there is an increasing proportion of people with sperm counts indicating subfertility or infertility.

"The high proportion of men from western countries with concentration below 40 million/ml is particularly concerning given the evidence that [sperm count] below this threshold is associated with a decreased monthly probability of conception," the authors wrote in the study.

It's not clear exactly what factors are to blame for the drop.

The paper notes that "environmental and lifestyle influences" are associated with lower sperm counts; particularly with regard to chemical exposure or maternal smoking affecting a child in the womb, or potentially exposure to pesticides in adult life.

While these issues might be part of the problem, scientists still aren't sure exactly what's causing the decline.

According to Richard Sharpe, a researcher unaffiliated with the new study, the newly published work "is about as close as we are going to get" to having hard evidence that sperm counts are declining in the Western world, he told The Guardian.

However, Sharpe also told The Guardian that the study doesn't actually speak to the root cause of the issue because we don't know what what issue is.

"This definitive study shows, for the first time, that this decline is strong and continuing."

"That is primarily because we have seriously under-invested in male reproductive research,” Sharpe said.

While we're not anywhere near a fertility disaster in the realm of The Handmaid's Tale, any new developments are still worth keeping an eye on in the future.

"Decreasing sperm count has been of great concern since it was first reported twenty-five years ago," Shanna Swan, one of the paper's authors, said in a statement.

"This definitive study shows, for the first time, that this decline is strong and continuing."

The results among men in the West contrast to studies in South America, Asia and Africa, which saw no significant decline in sperm counts. However, the authors said studies focusing on sperm counts in these regions are limited, therefore possibly skewing that conclusion.

"Our report of a continuing and robust decline should, therefore, trigger research into its causes, aiming for prevention," the authors wrote.